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https://github.com/diwu/LeetCode-Solutions-in-Swift

LeetCode Solutions in Swift 5
https://github.com/diwu/LeetCode-Solutions-in-Swift

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LeetCode Solutions in Swift 5

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![logo](https://github.com/diwu/ui-markdown-store/blob/master/swift_sample.png?raw=true)



Where it all begins: [https://github.com/apple/swift](https://github.com/apple/swift).



#### LeetCode Solutions in Swift 5

* Designed for your next job interview in Swift.

* Optimal solutions handpicked from the LeetCode community.

* Best time/space complexity guaranteed. (Think about **O(1) space** in Binary Tree Inorder Traversal. It's that good.)

* Written with Swift-specific language features in mind. (**guard**/**defer**, **map**/**filter**/**reduce**, **optionality**, **inout/@noescape**, **characters as extended grapheme clusters**, etc)

* Comprehensive test cases guarding against wrong answers and timeouts.

* A work in progress. Currently at **102** with **665** test cases.



Please note: Subscript-based O(1) time random access to characters inside a string is not supported by Swift, due to [the way Swift strings are stored](http://oleb.net/blog/2014/07/swift-strings/). Amortized O(1) time random access could be achieved if we manually slice the string and convert it into an array.



##### To run all test cases, launch the project and hit ⌘ + U.



##### Requires Xcode 12.4 (12D4e) or later.



1. [Two Sum](https://oj.leetcode.com/problems/two-sum/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_001_Two_Sum.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_001_Two_Sum.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_001_Two_Sum_Test.swift) - t=O(N), s=O(N) - Inspired by [@naveed.zafar](https://leetcode.com/discuss/10947/accepted-c-o-n-solution)

2. [Add Two Numbers](https://oj.leetcode.com/problems/add-two-numbers/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_002_Add_Two_Numbers.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_002_Add_Two_Numbers.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_002_Add_Two_Numbers_Test.swift) - t=O(N), s=O(1) - Inspired by [@potpie](https://leetcode.com/discuss/2308/is-this-algorithm-optimal-or-what)

3. [Longest Substring Without Repeating Characters ](https://oj.leetcode.com/problems/longest-substring-without-repeating-characters/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_003_Longest_Substring_Without_Repeating_Characters.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_003_Longest_Substring_Without_Repeating_Characters.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_003_Longest_Substring_Without_Repeating_Characters_Test.swift) - t=O(N), s=O(1) - Inspired by [@heiyanbin](https://leetcode.com/discuss/6168/my-o-n-solution)

4. [Median of Two Sorted Arrays](https://oj.leetcode.com/problems/median-of-two-sorted-arrays/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_004_Median_Of_Two_Sorted_Arrays.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Hard/ObjC_Hard_004_Median_Of_Two_Sorted_Arrays.m) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_004_Median_Of_Two_Sorted_Arrays_Test.swift) - t=O(log(min(m,n))), s=O(1) - Inspired by [@MissMary](https://leetcode.com/discuss/15790/share-my-o-log-min-m-n-solution-with-explanation) - Be sure to read [@MissMary](https://leetcode.com/discuss/15790/share-my-o-log-min-m-n-solution-with-explanation)'s [full explanation](https://leetcode.com/discuss/15790/share-my-o-log-min-m-n-solution-with-explanation) on how she achieved logarithmic time complexity with a few nice mathematic tricks.

5. [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_005_Longest_Palindromic_Substring.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_005_Longest_Palindromic_Substring.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_005_Longest_Palindromic_Substring_Test.swift) - t=O(N^2), s=O(1) - Inspired by [@hh1985](https://leetcode.com/discuss/32204/simple-c-solution-8ms-13-lines)

6. [ZigZag Conversion](https://oj.leetcode.com/problems/zigzag-conversion/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_006_ZigZag_Conversion.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_006_ZigZag_Conversion_Test.swift) - t=O(N), s=O(N) - Inspired by [@dylan_yu](https://leetcode.com/discuss/10493/easy-to-understand-java-solution)

7. [Reverse Integer ](https://oj.leetcode.com/problems/reverse-integer/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_007_Reverse_Integer.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_007_Reverse_Integer_Test.swift) - t=O(N), s=O(1) - Inspired by [@wshaoxuan](https://oj.leetcode.com/discuss/120/how-do-we-handle-the-overflow-case)

8. [String to Integer (atoi)](https://oj.leetcode.com/problems/string-to-integer-atoi/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_008_String_to_Integer_atoi.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_008_String_to_Integer_atoi_Test.swift) - t=O(N), s=O(1) - Inspired by [@yuruofeifei](https://leetcode.com/discuss/8886/my-simple-solution)

9. [Palindrome Number](https://oj.leetcode.com/problems/palindrome-number/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_009_Palindrome_Number.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_009_Palindrome_Number_Test.swift), t=O(N), s=O(1) - Inspired by [@hln9319](https://leetcode.com/discuss/12693/neat-ac-java-code-o-n-time-complexity)

10. [Regular Expression Matching](https://oj.leetcode.com/problems/regular-expression-matching/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_010_Regular_Expression_Matching.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_010_Regular_Expression_Matching_Test.swift) - DP, t=O(M\*N), s=O(M\*N) - Inspired by [@xiaohui7](https://leetcode.com/discuss/18970/concise-recursive-and-dp-solutions-with-full-explanation-in)

11. [Container With Most Water](https://leetcode.com/problems/container-with-most-water/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_011_Container_With_Most_Water.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_011_Container_With_Most_Water_Test.swift) - t=O(N), s=O(1) - Inspired by [@franticguy](https://leetcode.com/discuss/14610/very-simple-o-n-solution)

12. [Integer To Roman](https://leetcode.com/problems/integer-to-roman/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_012_Integer_To_Roman.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_012_Integer_To_Roman_Test.swift) - t=O(1), s=O(1) - Inspired by [@flytosky](https://leetcode.com/discuss/1208/how-to-improve-code)

13. [Roman To Integer](https://leetcode.com/problems/roman-to-integer/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_013_Roman_To_Integer.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_013_Roman_To_Integer_Test.swift) - t=O(N), s=O(1) - Inspired by [@makeittrue](https://leetcode.com/discuss/2369/solution-for-this-question-but-dont-know-there-any-easier-way)

14. [Longest Common Prefix](https://leetcode.com/problems/longest-common-prefix/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_014_Longest_Common_Prefix.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_014_Longest_Common_Prefix_Test.swift) - t=O(N\*M), s=O(1) - Inspired by [@garysui](https://leetcode.com/discuss/15755/optimal-solution)

15. [3Sum](https://leetcode.com/problems/3sum/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_015_3Sum.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_015_3Sum_Test.swift) - t=O(N^2), s=O(N) - Inspired by [@peerlessbloom](https://leetcode.com/discuss/10756/my-accepted-o-n-2-solution-without-hashmap)

16. [3Sum Closest](https://leetcode.com/problems/3sum-closest/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_016_3Sum_Closest.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_016_3Sum_Closest_Test.swift) - t=O(N^2), s=O(N) - Inspired by [@vaibhavatul47](https://leetcode.com/discuss/6438/a-n-2-solution-can-we-do-better)

17. [Letter Combinations of A Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_017_Letter_Combinations_Of_A_Phone_Number.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_017_Letter_Combinations_Of_A_Phone_Number.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_017_Letter_Combinations_Of_A_Phone_Number_Test.swift) - t=(3^N), s=O(3^N) - Inspired by [@lirensun](https://leetcode.com/discuss/24431/my-java-solution-with-fifo-queue)

18. [4Sum](https://leetcode.com/problems/4sum/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_018_4Sum.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_018_4Sum_Test.swift) - t=O(N^3), s=O(N) - Inspired by [@zhaohaoshu](https://leetcode.com/discuss/9141/any-better-solution-than-o-n-3)

19. [Remove Nth Node From End of List](https://leetcode.com/problems/remove-nth-node-from-end-of-list/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_019_Remove_Nth_Node_From_End_Of_List.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_019_Remove_Nth_Node_From_End_Of_List_Test.swift) - t=O(N), s=O(1) - Inspired by [@i](https://leetcode.com/discuss/1656/is-there-a-solution-with-one-pass-and-o-1-space)

20. [Valid Parentheses](https://leetcode.com/problems/valid-parentheses/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_020_Valid_Parentheses.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_020_Valid_Parentheses_Test.swift) - t=O(N), s=O(N) - Inspired by [@exodia](https://leetcode.com/discuss/21440/sharing-my-simple-cpp-code-with-2ms)

21. [Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_021_Merge_Two_Sorted_Lists.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_021_Merge_Two_Sorted_Lists_Test.swift) - t=O(max(M,N)), s=O(1) - Inspired by [@xiaohui7](https://leetcode.com/discuss/18986/14-line-clean-c-solution)

22. [Generate Parentheses](https://leetcode.com/problems/generate-parentheses/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_022_Generate_Parentheses.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_022_Generate_Parentheses_Test.swift) - t=O(N^2), s=O(N^2) - Inspired by [@klyc0k](https://leetcode.com/discuss/14436/concise-recursive-c-solution)

23. [Merge K Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_023_Merge_K_Sorted_Lists.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_023_Merge_K_Sorted_Lists_Test.swift) - t=O(N\*logK), s=O(logK), where where N is the total number of elements, K is the number of lists - Inspired by [@wksora](https://leetcode.com/discuss/9279/a-java-solution-based-on-priority-queue)

24. [Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_024_Swap_Nodes_In_Pairs.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_024_Swap_Nodes_In_Pairs_Test.swift), t=O(N), s=O(1) - Inspired by [@mike3](https://leetcode.com/discuss/3608/seeking-for-a-better-solution)

25. [Reverse Nodes in k-Group](https://leetcode.com/problems/reverse-nodes-in-k-group/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_025_Reverse_Nodes_In_K_Group.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_025_Reverse_Nodes_In_K_Group_Test.swift) - t=O(N), s=O(1) - Inspired by [@sean hyuntaek](https://leetcode.com/discuss/6113/my-solution-accepted-in-java) and [@shpolsky](https://leetcode.com/discuss/21301/short-but-recursive-java-code-with-comments)

26. [Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_026_Remove_Duplicates_from_Sorted_Array.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_026_Remove_Duplicates_from_Sorted_Array_Test.swift) - t=O(N), s=O(1) - Inspired by [@liyangguang1988](https://leetcode.com/discuss/10314/my-solution-time-o-n-space-o-1)

27. [Remove Element](https://leetcode.com/problems/remove-element/)- Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_027_Remove_Element.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_027_Remove_Element_Test.swift) - t=O(N), s=O(1) - Inspired by [@daxianji007](https://leetcode.com/discuss/3753/my-solution-for-your-reference)

28. [Implement strStr()](https://leetcode.com/problems/implement-strstr/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_028_Implement_StrStr.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_028_Implement_StrStr_Test.swift) - KMP: t=O(M+N), s=O(N). Brute-force: t=O(M\*N), s=O(1) - Brute-force inspired by [@shichaotan](https://leetcode.com/discuss/19962/a-very-clean-solution-brute-force), KMP inspired by [@zhenhai](https://leetcode.com/discuss/11814/accepted-kmp-solution-in-java-for-reference).

29. [Divide Two Integers](https://leetcode.com/problems/divide-two-integers/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_029_Divide_Two_Integers.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_029_Divide_Two_Integers_Test.swift) - t=O((logN)^2), s=O(1) - Inspired by [@lucastan & @ngcl](https://leetcode.com/discuss/11358/simple-o-log-n-2-c-solution)

30. [Substring with Concatenation of All Words](https://leetcode.com/problems/substring-with-concatenation-of-all-words/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_030_Substring_With_Concatenation_Of_All_Words.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_030_Substring_With_Concatenation_Of_All_Words_Test.swift) - t=O(N\*W), s=O(L\*W), where N is the length of the string, W is the length of a word, L is the length of the words list. - Inspired by [@ramakanthd92](https://leetcode.com/discuss/366/better-solution-than-brute-force)

31. [Next Permutation](https://leetcode.com/problems/next-permutation/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_031_Next_Permutation.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_031_Next_Permutation_Test.swift) - t=O(N), s=O(1) - Inspired by [@yuyibestman](https://leetcode.com/discuss/8472/share-my-o-n-time-solution)

32. [Longest Valid Parentheses](https://leetcode.com/problems/longest-valid-parentheses/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_032_Longest_Valid_Parentheses.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_032_Longest_Valid_Parentheses_Test.swift) - One pass, t=O(N), s=O(N) - Inspired by [@jerryrcwong](https://leetcode.com/discuss/8092/my-dp-o-n-solution-without-using-stack)

33. [Search in Rotated Sorted Array](https://leetcode.com/problems/search-in-rotated-sorted-array/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_033_Search_In_Rotated_Sorted_Array.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_033_Search_In_Rotated_Sorted_Array_Test.swift) - t=O(logN), s=O(1) - Inspired by [@sean hyuntaek](https://leetcode.com/discuss/5707/algorithm-using-binary-search-accepted-some-suggestions-else)

34. [Search for a Range](https://leetcode.com/problems/search-for-a-range/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_034_Search_For_A_Range.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_034_Search_For_A_Range_Test.swift) - t=O(logN), s=O(1)

35. [Search Insert Position](https://leetcode.com/problems/search-insert-position/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_035_Search_Insert_Position.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_035_Search_Insert_Position_Test.swift) - t=O(logN), s=O(1)

36. [Valid Sudoku](https://leetcode.com/problems/valid-sudoku/) - Medium - [Swift Solution](./Solutions/Solutions/Easy/Easy_036_Valid_Sudoku.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_036_Valid_Sudoku_Test.swift) - t=O(1), s=O(1)

37. [Sudoku Solver](https://leetcode.com/problems/sudoku-solver/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_037_Sudoku_Solver.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_037_Sudoku_Solver_Test.swift) - DFS

38. [Count and Say](https://leetcode.com/problems/count-and-say/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_038_Count_And_Say.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_038_Count_And_Say_Test.swift) - t=(2^N), s=O(2^N)

39. [Combination Sum](https://leetcode.com/problems/combination-sum/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_039_Combination_Sum.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_039_Combination_Sum_Test.swift) - t=O(N*M), s=O(M), where N is the length of the candidates array, M is the value of the target integer.

40. [Combination Sum II](https://leetcode.com/problems/combination-sum-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_040_Combination_Sum_II.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_040_Combination_Sum_II_Test.swift) - t=O(N*M), s=O(M), where N is the length of the candidates array, M is the value of the target integer.

41. [First Missing Positive](https://leetcode.com/problems/first-missing-positive/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_041_First_Missing_Positive.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_041_First_Missing_Positive_Test.swift) - t=O(N), s=O(1)

42. [Trapping Rain Water](https://leetcode.com/problems/trapping-rain-water/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_042_Trapping_Rain_Water.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_042_Trapping_Rain_Water_Test.swift) - t=O(N), s=O(1)

43. [Multiply Strings](https://leetcode.com/problems/multiply-strings/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_043_Multiply_Strings.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_043_Multiply_Strings_Test.swift) - t=O(N\*M), s=O(N+M)

44. [Wildcard Matching](https://leetcode.com/problems/wildcard-matching/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_044_Wildcard_Matching.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_044_Wildcard_Matching_Test.swift) - t=O(N), s=O(1)

45. [Jump Game II](https://leetcode.com/problems/jump-game-ii/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_045_Jump_Game_II.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_045_Jump_Game_II_Test.swift) - t=O(N), s=O(1)

46. [Permutations](https://leetcode.com/problems/permutations/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_046_Permutations.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_046_Permutations_Test.swift) - t=O(N!), s=O(N!)

47. [Permutations II](https://leetcode.com/problems/permutations-ii/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_047_Permutations_II.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_047_Permutations_II_Test.swift) - t=O(N!), s=O(N!)

48. [Rotate Image](https://leetcode.com/problems/rotate-image/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_048_Rotate_Image.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_048_Rotate_Image_Test.swift) - t=O(N\*N), s=O(1)

49. [Anagrams](https://leetcode.com/problems/anagrams/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_049_Anagrams.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_049_Anagrams_Test.swift) - t=O(N), s=O(N)

50. [Pow(x, n)](https://leetcode.com/problems/powx-n/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_050_Pow_X_N.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_050_Pow_X_N_Test.swift) - t=O(logN), s=O(logN)

51. [N-Queens](https://leetcode.com/problems/n-queens/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_051_N_Queens.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_051_N_Queens_Test.swift) - t=O(N^2), s=O(N^2)

52. [N-Queens II](https://leetcode.com/problems/n-queens-ii/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_052_N_Queens_II.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_052_N_Queens_II_Test.swift) - t= well it's complicated...

53. [Maximum Subarray](https://leetcode.com/problems/maximum-subarray/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_053_Maximum_Subarray.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_053_Maximum_Subarray_Test.swift) - t=O(N), s=O(1)

54. [Spiral Matrix](https://leetcode.com/problems/spiral-matrix/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_054_Spiral_Matrix.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_054_Spiral_Matrix_Test.swift) - t=O(N), s=O(N)

55. [Jump Game](https://leetcode.com/problems/jump-game/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_055_Jump_Game.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_055_Jump_Game_Test.swift) - t=O(N), s=O(1)

56. [Merge Intervals](https://leetcode.com/problems/merge-intervals/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_056_Merge_Intervals.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_056_Merge_Intervals_Test.swift) - t=O(N\*logN), s=O(N)

57. [Insert Interval](https://leetcode.com/discuss/3971/in-place-solution-ask-for-suggestion) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_057_Insert_Interval.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_057_Insert_Interval_Test.swift) - t=O(N), s=O(N)

58. [Length of Last Word](https://leetcode.com/problems/length-of-last-word/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_058_Length_Of_Last_Word.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_058_Length_Of_Last_Word_Test.swift) - t=O(N), s=O(1)

59. [Spiral Matrix II](https://leetcode.com/problems/spiral-matrix-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_059_Spiral_Matrix_II.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_059_Spiral_Matrix_II_Test.swift) - t=O(N^2), s=O(N^2)

60. [Permutation Sequence](https://leetcode.com/problems/permutation-sequence/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_060_Permutation_Sequence.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_060_Permutation_Sequence_Test.swift) - t=O(N^2), s=O(N)

61. [Rotate List](https://leetcode.com/problems/rotate-list/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_061_Rotate_List.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_061_Rotate_List_Test.swift) - t=O(N), s=O(1)

62. [Unique Paths](https://leetcode.com/problems/unique-paths/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_062_Unique_Paths.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_062_Unique_Paths_Test.swift) - t=O(min(M, N)), s=O(1)

63. [Unique Paths II](https://leetcode.com/problems/unique-paths-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_063_Unique_Paths_II.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_063_Unique_Paths_II_Test.swift) - t=O(M\*N), s=O(1)

64. [Minimum Path Sum](https://leetcode.com/problems/minimum-path-sum/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_064_Minimum_Path_Sum.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_064_Minimum_Path_Sum_Test.swift) - t=O(M\*N), s=O(1)

65. [Valid Number](https://leetcode.com/problems/valid-number/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_065_Valid_Number.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_065_Valid_Number_Test.swift) - t=O(N), s=O(1)

66. [Plus One](https://leetcode.com/problems/plus-one/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_066_Plus_One.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_066_Plus_One_Test.swift) - t=O(N), s=O(1)

67. [Add Binary](https://leetcode.com/problems/add-binary/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_067_Add_Binary.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_067_Add_Binary_Test.swift) - t=O(max(M,N)), s=O(max(M,N))

68. [Text Justification](https://leetcode.com/problems/text-justification/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_068_Text_Justification.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_068_Text_Justification_Test.swift) - t<=O(N\*L), s<=O(N\*L), where N is the length of the input words array, L is the required justified length

69. [Sqrt(x)](https://leetcode.com/problems/sqrtx/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_069_Sqrt_X.swift) - [Test Cases](https://github.com/diwu/LeetCode-Solutions-in-Swift/blob/master/Solutions/SolutionsTests/Medium/Medium_069_Sqrt_X_Test.swift) - t=O(logN), s=O(1)

70. [Climbing Stairs](https://leetcode.com/problems/climbing-stairs/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_070_Climbing_Stairs.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_070_Climbing_Stairs_Test.swift) - t=O(N), s=O(1)

71. [Simplify Path](https://leetcode.com/problems/simplify-path/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_071_Simplify_Path.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_071_Simplify_Path_Test.swift) - t=O(N), s=O(N)

72. [Edit Distance](https://leetcode.com/problems/edit-distance/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_072_Edit_Distance.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_072_Edit_Distance_Test.swift) - t=O(M\*N), s=O(min(M,N))

73. [Set Matrix Zeroes](https://leetcode.com/problems/set-matrix-zeroes/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_073_Set_Matrix_Zeroes.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_073_Set_Matrix_Zeroes_Test.swift) - t=O(M\*N), s=O(1)

74. [Search a 2D Matrix](https://leetcode.com/problems/search-a-2d-matrix/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_074_Search_A_2D_Matrix.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_074_Search_A_2D_Matrix_Test.swift) - t=O(logN), s=O(1)

75. [Sort Colors](https://leetcode.com/problems/sort-colors/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_075_Sort_Colors.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_075_Sort_Colors_Test.swift) - t=O(N), s=O(1), one pass

76. [Minimum Window Substring](https://leetcode.com/problems/minimum-window-substring/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_076_Minimum_Window_Substring.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_076_Minimum_Window_Substring_Test.swift) - t=O(N), s=O(N)

77. [Combinations](https://leetcode.com/problems/combinations/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_077_Combinations.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_077_Combinations_Test.swift) - t=O(Combination(n, k)+2^(k-1)), s is complicated.

78. [Subsets](https://leetcode.com/problems/subsets/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_078_Subsets.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_078_Subsets_Test.swift) - t=O(N\*2^N), s=O(n\*2^N)

79. [Word Search](https://leetcode.com/problems/word-search/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_079_Word_Search.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_079_Word_Search_Test.swift) - t=O(M\*N\*L), s=O(L), where L is the length of the word.

80. [Remove Duplicates from Sorted Array II](https://leetcode.com/problems/remove-duplicates-from-sorted-array-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_080_Remove_Duplicates_From_Sorted_Array_II.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_080_Remove_Duplicates_From_Sorted_Array_II_Test.swift) - t=O(N), s=O(1)

81. [Search in Rotated Sorted Array II](https://leetcode.com/problems/search-in-rotated-sorted-array-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_081_Search_In_Rotated_Sorted_Array_II.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_081_Search_In_Rotated_Sorted_Array_II_Test.swift) - average t=O(logN), worst case t=O(N), s=O(1)

82. [Remove Duplicates from Sorted List II](https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_082_Remove_Duplicates_From_Sorted_List_II.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_082_Remove_Duplicates_From_Sorted_List_II_Test.swift) - t=O(N), s=O(1)

83. [Remove Duplicates from Sorted List](https://leetcode.com/problems/remove-duplicates-from-sorted-list/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_083_Remove_Duplicates_From_Sorted_List.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_083_Remove_Duplicates_From_Sorted_List_Test.swift) - t=O(N), s=O(1) - Inspired by [@Tao2014](https://leetcode.com/discuss/7188/concise-solution-and-memory-freeing)

84. [Largest Rectangle in Histogram](https://leetcode.com/problems/largest-rectangle-in-histogram/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_084_Largest_Rectangle_In_Histogram.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_084_Largest_Rectangle_In_Histogram_Test.swift) - t=O(N), s=O(N) - Inspired by [geeksforgeeks](http://www.geeksforgeeks.org/largest-rectangle-under-histogram/)

85. [Maximal Rectangle](https://leetcode.com/problems/maximal-rectangle/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_085_Maximal_Rectangle.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_085_Maximal_Rectangle_Test.swift) - t=O(M\*N), s=O(min(M,N)) - Inspired by [@morrischen2008](https://leetcode.com/discuss/20240/share-my-dp-solution)

86. [Partition List](https://leetcode.com/problems/partition-list/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_086_Partition_List.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_086_Partition_List_Test.swift) - t=O(N), s=O(1) - Inspired by [@shichaotan](https://leetcode.com/discuss/21032/very-concise-one-pass-solution)

87. [Scramble String](https://leetcode.com/problems/scramble-string/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_087_Scramble_String.swift) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_087_Scramble_String_Test.swift) - time and space complexity need further investigation - Inspired by [@Charles_p](https://leetcode.com/discuss/3632/any-better-solution) and [@dtx0](https://leetcode.com/discuss/3632/any-better-solution)

88. [Merge Sorted Array](https://leetcode.com/problems/merge-sorted-array/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_088_Merge_Sorted_Array.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_088_Merge_Sorted_Array_Test.swift) - t=O(N), s=O(1) - Inspired by [@leetchunhui](https://leetcode.com/discuss/8233/this-is-my-ac-code-may-help-you)

89. [Gray Code](https://leetcode.com/problems/gray-code/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_089_Gray_Code.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_089_Gray_Code_Test.swift) - t=O(2^N), s=O(2^N) - Inspired by [@jinrf](https://discuss.leetcode.com/topic/8557/an-accepted-three-line-solution-in-java)

90. [Subsets II](https://leetcode.com/problems/subsets-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_090_Subsets_II.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_090_Subsets_II_Test.swift) - t=O(2^N), s=O(2^N) - Inspired by [@mathsam](https://leetcode.com/discuss/14902/c-solution-and-explanation)

91. [Decode Ways](https://leetcode.com/problems/decode-ways/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_091_Decode_Ways.swift) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_091_Decode_Ways_Test.swift) - t=O(N), s=O(N) - Inspired by [@manky](https://leetcode.com/discuss/8527/dp-solution-java-for-reference)

92. [Reverse Linked List II](https://leetcode.com/problems/reverse-linked-list-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_092_Reverse_Linked_List_II.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_092_Reverse_Linked_List_II.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_092_Reverse_Linked_List_II_Test.swift) - t=O(N), s=O(1), One Pass - Inspired by [@ardyadipta](https://leetcode.com/discuss/25580/simple-java-solution-with-clear-explanation)

93. [Restore IP Addresses](https://leetcode.com/problems/restore-ip-addresses/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_093_Restore_IP_Addresses.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_093_Restore_IP_Addresses.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_093_Restore_IP_Addresses_Test.swift) - t=O(1), s=O(1) - Inspired by [@fiona_mao](https://leetcode.com/discuss/12790/my-code-in-java)

94. [Binary Tree Inorder Traversal](https://leetcode.com/problems/binary-tree-inorder-traversal/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_094_Binary_Tree_Inorder_Traversal.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_094_Binary_Tree_Inorder_Traversal.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_094_Binary_Tree_Inorder_Traversal_Test.swift) - Recursion and Iteration t=O(N), average s=O(log(N)), worst s=O(N) - Morris t=O(N), both average & worst s=O(1) - The way we achieved s=O(1) is pretty impressive. Be sure to read the Morris algorithm - Inspired by [@lvlolitte](https://leetcode.com/discuss/19765/iterative-solution-in-java-simple-and-readable) and [@blue_y](https://leetcode.com/discuss/11295/morris-traversal-no-recursion-no-stack)

95. [Unique Binary Search Trees II](https://leetcode.com/problems/unique-binary-search-trees-ii/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_095_Unique_Binary_Search_Trees_II.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_095_Unique_Binary_Search_Trees_II.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_095_Unique_Binary_Search_Trees_II_Test.swift) - t=O(N^(N-1)), s=I've no idea - Inspired by [@Jayanta](https://leetcode.com/discuss/10254/a-simple-recursive-solution)

96. [Unique Binary Search Trees](https://leetcode.com/problems/unique-binary-search-trees/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_096_Unique_Binary_Search_Trees.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_096_Unique_Binary_Search_Trees.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_096_Unique_Binary_Search_Trees_Test.swift) - t=O(N^2), s=O(N) -  Inspired by [@liaison](https://leetcode.com/discuss/24282/dp-solution-in-6-lines-with-explanation-f-i-n-g-i-1-g-n-i)

97. [Interleaving String](https://leetcode.com/problems/interleaving-string/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_097_Interleaving_String.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Hard/ObjC_Hard_097_Interleaving_String.m) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_097_Interleaving_String_Test.swift) - t=O(N\*M), s=O(N\*M) - Inspired by [@sherryxmhe](https://leetcode.com/discuss/11694/my-dp-solution-in-c)

98. [Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/) - Medium - [Swift Solution](./Solutions/Solutions/Medium/Medium_098_Validate_Binary_Search_Tree.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Medium/ObjC_Medium_098_Validate_Binary_Search_Tree.m) - [Test Cases](./Solutions/SolutionsTests/Medium/Medium_098_Validate_Binary_Search_Tree_Test.swift) - t=O(N), average s=O(logN), worst s=O(N) - Inspired by [@jakwings](https://leetcode.com/discuss/14886/order-traversal-please-rely-buggy-int_max-int_min-solutions)

99. [Recover Binary Search Tree](https://leetcode.com/problems/recover-binary-search-tree/) - Hard - [Swift Solution](./Solutions/Solutions/Hard/Hard_099_Recover_Binary_Search_Tree.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Hard/ObjC_Hard_099_Recover_Binary_Search_Tree.m) - [Test Cases](./Solutions/SolutionsTests/Hard/Hard_099_Recover_Binary_Search_Tree_Test.swift) - t=O(N), s=(1) - Inspired by [@siyang3](https://leetcode.com/discuss/26310/detail-explain-about-morris-traversal-finds-incorrect-pointer)

100. [Same Tree](https://leetcode.com/problems/same-tree/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_100_Same_Tree.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Easy/ObjC_Easy_100_Same_Tree.m) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_100_Same_Tree_Test.swift) - t=O(N), average s=O(logN), worst s=O(N) - Inspired by [@JohnWeiGitHub](https://leetcode.com/discuss/3470/seeking-for-better-solution) and [@scott](https://leetcode.com/discuss/22197/my-non-recursive-method)

101. [Symmetric Tree](https://leetcode.com/problems/symmetric-tree/) - Easy - [Swift Solution](./Solutions/Solutions/Easy/Easy_101_Symmetric_Tree.swift) - [ObjC Solution](./Solutions/Solutions_ObjC/Easy/ObjC_Easy_101_Symmetric_Tree.m) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_101_Symmetric_Tree_Test.swift) - t=O(N), best s=O(logN), worst s=O(N) - Inspired by [@xuanaux](https://leetcode.com/discuss/456/recusive-solution-symmetric-optimal-solution-inordertraversal)

102. [Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/) - [Swift Solution](./Solutions/Solutions/Easy/Easy_102_Binary_Tree_Level_Order_Traversal.swift) - [Test Cases](./Solutions/SolutionsTests/Easy/Easy_102_Binary_Tree_Level_Order_Traversal_Test.swift) - t=O(N), best s=O(logN), worst s=O(N) - Inspired by [@d40a](https://discuss.leetcode.com/topic/2399/it-is-my-ac-solution)



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