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https://github.com/NikiforovAll/leetcode-playground

Leetcode solutions
https://github.com/NikiforovAll/leetcode-playground

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  Leetcode 👨‍💻




# Leetcode-playground



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Leetcode url: 



| # | Title | Solution | Basic idea (One line) |

|---| ----- | -------- | --------------------- |

| 1 | [Two Sum](https://leetcode.com/problems/two-sum/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/1.two-sum.cs) | 1. Hash O(n) and O(n) space.
2. Sort O(n * log n) and search with two points O(n) and O(1) space.|

| 2 | [AddTwoNumbers](https://leetcode.com/problems/add-two-numbers/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/2.add-two-numbers.cs) | Traverse single-linked list and do modulo 10 sum |

| 15 | [3Sum](https://leetcode.com/problems/3sum/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/15.3-sum.cs) | Sort initial array, loop over array and pick current element, solve two-sum problem based on two pointers method for array to the right of current element with *target* equals to negated value of current element|

| 26 | [RemoveDuplicatesFromSortedArray](https://leetcode.com/articles/remove-duplicates-from-sorted-array/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/26.remove-duplicates-from-sorted-array.cs) | 1. Two pointers, slow/fast runner approach |

| 27 | [RemoveElement](https://leetcode.com/problems/remove-element/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/27.remove-element.cs) | Two pointers, skip element at the end and swap at the start.  |

| 35 | [SerachInsertPosition](https://leetcode.com/problems/search-insert-position/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/35.search-insert-position.cs) | 1. Two pointers, skip element at the end and swap at the start.  2. Two pointers, both start at the beginning result-sub-list pointer and full-scan pointer |

| 69 | [SqrtX](https://leetcode.com/problems/sqrtx/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/src\LeetCodeSolutions\69.sqrt-x.cs) | Bisection, lazy curr element calculation f(mid) < x < f(mid+1), use division instead of multiplication to speed up calculation |

| 75 | [SortColors](https://leetcode.com/problems/sort-colors/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/src\LeetCodeSolutions\75.sort-colors.cs) | Two pointers approach, fill the rest of the array with 1 |

| 88 | [MergeSortedArray](https://leetcode.com/problems/3sum/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/88.merge-sorted-array.cs) | 1. Two pointers solution, store pointers to latest not processed in nums1, nums2 (=p,q). Shift nums1[pos..pos+m-p]->1 when item from nums2 is written and update p,q 2. Two pointers solution, start from the end to avoid shift operation|

| 96 | [UniqueBinarySearchTrees](https://leetcode.com/problems/unique-binary-search-trees) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/96.unique-binary-search-trees.cs) | Dynamic programming based on catalan number calculation. The goal is to fix one number as root element and calculate number of unique binary trees based on multiplied combinations of subtrees T\[i\]*T\[i - j -1\]|

| 130 | [SurroundedRegions](https://leetcode.com/problems/surrounded-regions) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/130.surrounded-regions.cs) | Traverse inner part of board and use dfs to check potential candidates for checking, commit if it is a valid group |

| 189 | [RotateArray](https://leetcode.com/problems/rotate-array) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/189.rotate-array.cs) | 1. Shifting with additional O(k) memory  2. Reverse in-place based on pivot element  3. Cyclic replacement |

| 200 | [NumberOfIslands](https://leetcode.com/problems/number-of-islands) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/200.number-of-islands.cs) | In-place uncheck of not visited islands via DFS/BFS |

| 222 | [CountCompleteTreeNodes](https://leetcode.com/problems/count-complete-tree-nodes) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/src\LeetCodeSolutions\222.count-complete-tree-nodes.cs) | Usage of property of complete tree, depending on height of subtree we could determine the part of tree under construction |

| 231 | [PowerOfTwo](https://leetcode.com/problems/power-of-two) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/231.power-of-two.cs) | Bit manipulation |

| 275 | [HIndexII](https://leetcode.com/problems/h-index-ii) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/275.h-index-ii.cs) | Binary search for first A\[mid\] >= N - mind, return number of elements to the right of the pointer |

| 278 | [FirstBadVersion](https://leetcode.com/problems/first-bad-version) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/278.first-bad-version.cs) | Bisection |

| 380 | [InsertDeleteGetRandom1](https://leetcode.com/problems/insert-delete-getrandom-o1/description/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/380.insert-delete-get-random-o-1.cs) | Maintain map(val -> ind in array) + list, removal is based on swap of last element |

| 414 | [ThirdMaximumNumber](https://leetcode.com/problems/third-maximum-number/description/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/414.third-maximum-number.cs) | Three consecutive counters, stored element shift |

| 485 | [ValidateIPAddress](https://leetcode.com/problems/validate-ip-address/description/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/468.validate-ip-address.cs) | Validation pipeline with the list of heuristics |

| 485 | [MaxConsecutiveOnes](https://leetcode.com/problems/third-maximum-number/description/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/485.max-consecutive-ones.cs) | Sliding window for non-zero part of array and current sum calculation as index diff |

| 518 | [CoinChange2](https://leetcode.com/problems/coin-change-2) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/518.coin-change-2.cs) | Classic DP problem, cache total combinations of previous subproblems in a list |

| 547 | [FriendCircles](https://leetcode.com/problems/friend-circles) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/547.friend-circles.cs) | Checked DFS nodes |

| 904 | [FruitIntoBaskets](https://leetcode.com/problems/fruit-into-baskets) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/904.fruit-into-baskets.cs) | Sliding window, a bunch of pointers to store consecutive sequence of previous latest element |

| 977 | [SquaresOfASortedArray](https://leetcode.com/problems/squares-of-a-sorted-array) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/977.squares-of-a-sorted-array.cs) | 1. Merge sort, array with negative numbers is traversed in a reversed manner  2. Two pointers |

| 1004 | [MaxConsecutiveOnesIII](https://leetcode.com/problems/max-consecutive-ones-iii) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/1004.max-consecutive-ones-iii.cs) | Sliding window, remove elements from the tail |

| 1004 | [TwoCityScheduling](https://leetcode.com/problems/two-city-scheduling) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/1029.two-city-scheduling.cs) | Greedy solution based on costs difference heuristic + sorting |

| 1114 | [DuplicateZeros](https://leetcode.com/problems/duplicate-zeros/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/1089.duplicate-zeros.cs) | Implement array shift starting from the end for each zero element, don't forget to skip updated zero |

| 1114 | [FirstInOrder](https://leetcode.com/problems/print-in-order/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/1114.print-in-order.cs) | Event-based blocking ManualResetEventSlim |

| 1115 | [PrintFooBarAlternately](https://leetcode.com/problems/print-foobar-alternately/description/) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/1115.print-foo-bar-alternately.cs) | Event-based blocking ManualResetEventSlim |

| 1295 | [FindNumbersWithEvenNumberOfDigits](https://leetcode.com/problems/find-numbers-with-even-number-of-digits) | [C#](https://github.com/NikiforovAll/leetcode-playground/blob/master/src/LeetCodeSolutions/1295.find-numbers-with-even-number-of-digits.cs) | Reduce arr item to double digit by dividing by 100 and count |



## Other Leetcode Repos



1. 

2. [haoel's leetcode](https://github.com/haoel/leetcode)

3. [soulmachine's leetcode](https://github.com/soulmachine/leetcode)

4. [gouthampradhan's leetcode](https://github.com/gouthampradhan/leetcode)

5. [qiyuangong's leetcode](https://github.com/qiyuangong/leetcode)