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ProjectEuler\n\nProjectEuler practice\n\n## Why project euler project\n\n이걸 안건 2017년 쯤이었던 것 같다.\n영국에서 프로그래머 활동을 하는 어떤 분의 블로그를 봤는데, 꾸준히 그리고 부지런히 프로젝트 오일러의 문제 풀이를 하고 있는 걸 봤다. 그때는 막연히 나도 하면 좋겠다는 생각만 했을 뿐 실천해야 겠다는 생각은 하지 못했다. 그러다가 실천 의지를 발휘해서 하기로 했다.\n\n### 그 분의 블로그 주소\n\n[http://ntalbs.github.io/](http://ntalbs.github.io/)\n\n## How to pick programming language\n\n### Before\n\n[TIOBE](https://www.tiobe.com/tiobe-index/) 순위 중 1 - 5위 까지의 프로그래밍 언어를 랜덤으로 하나씩 선택 해 문제를 풀고, 다음 문제는 나머지 선택되지 않은 프로그래밍 언어 중 랜덤하게 선택해서 풀어 나가는 방식\n\n### Now\n\n[2020 state of the octoverse](https://octoverse.github.com/)에서 Top languages over the years 순위를 참고해 1 - 10위 까지의 프로그래밍 언어를 랜덤으로 하나씩 선택해 풀고, 다음 문제는 나머지 선택되지 않은 프로그래밍 언어 중 랜덤하게 선택해서 풀어 나가는 방식.\n\n26번 문제부터 이 방식을 채택해서 진행함.\n\n2020 top languages over the years는 다음과 같다.\n\n![TopLanguagesOverTheYears_2020](TopLanguagesOverTheYears_2020.png)\n\n## Solve sequence, using software engineering\n\n### Analysis\n\n- 실제 알고 있어야 하는 수학적인 개념\n- 수학적인 풀이 예제를 통한 구현 방식 이해\n\n### Design\n\n- 생각한 것을 pseudo level로 풀어야 하는 언어에 맞게 정리\n- 생각했던 알고리즘이나 만들어야 하는 함수를 문서로 작성\n\n### Coding\n\n- 분석, 설계 했던 내용 그대로 코딩 진행\n- 코딩 진행 중 생각했던 것과 달랐을 경우 문서 내용에 추가\n\n### Test\n\n- 실제 테스트 데이터나 코드는 작성하지 않음\n- 필요에 따라 성능 테스트를 위한 시간 측정이나 logging은 진행 =\u003e 필요시 uncomment\n\n### Maintenance\n\n- 더 나은 구현 방식에 대한 걸 찾으면 diff 코드를 문서에 추가\n- 언어별 다른 점이나 더 나은점이 있다면 역시 변경하면서 문서에 추가\n- Revised할 문제가 필요하다면 별도로 새 프로젝트로 만들어서 진행\n\n### 중요한 것!\n\n- 절대 코딩부터 하지 말고 문제 자체를 이해한다.\n- 주어진 문제 내애서 해결해야 하는 방안을 코드로 구현할 수 있게 설계를 진행하고 문서 작성을 함\n- 코딩은 그 다음에 해도 늦지 않음\n\n## Notes\n\n- 문제를 푸는데 필요한 수학적 개념을 따로 정리\n- 링크 참고: [Notes](/Notes.md)\n\n## Problems\n\n### [Problem 27, Quadratic primes](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem27)\n\n- Top languages over the years 2020: picked programming is JavaScript (top 1)\n\n### [Problem 26, Reciprocal cycles](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem26)\n\n- Top languages over the years 2020: picked programming is C# (top 5)\n\n### [Problem 1 revised, Multiples 3 and 5](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem1_revised)\n\n- TIOBE index for Jun 2018, Ranking Top 1: Java\n\n### [Problem 25, 1000-digit Fibonacci number](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem25)\n\n- TIOBE index for October 2020, Ranking Top 1: C\n\n### [Problem 24, Lexicographic permutations](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem24)\n\n- TIOBE index for September 2020, Ranking Top 3: Python\n\n### [Problem 23, Non-abundant sums](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem23)\n\n- TIOBE index for July 2020, Ranking Top 4: C++\n\n### [Problem 19 revised, Counting Sundays](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem19_revised)\n\n- TIOBE index for December 2019, Ranking Top 5: C#\n\n### [Problem 22, Names scores](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem22)\n\n- TIOBE index for June 2020, Ranking Top 2: Java\n\n### [Problem 21, Amicable numbers](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem21)\n\n- TIOBE index for May 2020, Ranking Top 5: C#\n\n### [Problem 17 revised, Number letter counts](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem17_revised)\n\n- TIOBE index for September 2019, Ranking Top 1: Java\n\n### [Problem 20, Factorial digit sum](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem20)\n\n- TIOBE index for February 2020, Ranking Top 2: C\n\n### [Problem 19, Counting Sundays](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem19)\n\n- TIOBE index for December 2019, Ranking Top 5: C#\n\n### [Problem 18, Maximum path sum I](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem18)\n\n- TIOBE index for October 2019, Ranking Top 4: C++\n\n### [Problem 17, Number letter counts](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem17)\n\n- TIOBE index for September 2019, Ranking Top 1: Java\n\n### [Problem 16, Power digit sum](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem16)\n\n- TIOBE index for December 2018, Ranking Top 3: Python\n\n### [Problem 15, Lattice paths](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem15)\n\n- TIOBE index for September 2018, Ranking Top 2: C\n\n### [Problem 14, Longest Collatz sequence](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem14)\n\n- TIOBE index for July 2019, Ranking Top 4: C++\n\n### [Problem 13, Large Sum](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem13)\n\n- TIOBE index for January 2019, Ranking Top 5: Visual Basic .NET\n\n### [Problem 12, Highly divisible triangular number](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem12)\n\n- TIOBE index for September 2018, Ranking Top 1: Java\n\n### [Problem 11, Largest product in a grid](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem11)\n\n- TIOBE index for August 2018, Ranking Top 3: C++\n\n### [Problem 10, Summation of primes](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem10)\n\n- TIOBE index for August 2018, Ranking Top 5: Visual Basic .NET\n\n### [Problem 9, Special Pythagorean triplet](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem9)\n\n- TIOBE index for August 2018, Ranking Top 2: C\n\n### [Problem 8, Largest product in a series](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem8)\n\n- TIOBE index for August 2018, Ranking Top 1: Java\n\n### [Problem 7, 10001st prime](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem7)\n\n- TIOBE index for August 2018, Ranking Top 4: Python\n\n### [Problem 6, Sum square difference](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem6)\n\n- TIOBE index for August 2018, Ranking Top 3: C++\n\n### [Problem 5, Smallest multiple](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem5)\n\n- TIOBE index for August 2018, Ranking Top 3: C++\n\n### [Problem 4, Largest palindrome product](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem4)\n\n- TIOBE index for July 2018, Ranking Top 2: C\n\n### [Problem 3, Largest prime factor](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem3)\n\n- TIOBE index for July 2018, Ranking Top 5: Visual Basic .NET\n\n### [Problem 2, Even Fibonacci numbers](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem2)\n\n- TIOBE index for July 2018, Ranking Top 4: Python\n\n### [Problem 1, Multiples 3 and 5](https://github.com/jongfeel/ProjectEuler/tree/master/Problems/Problem1)\n\n- TIOBE index for Jun 2018, Ranking Top 1: 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