Merge pull request #1461 from cirosantilli/patch-2

problem-sets: leetcode

problem-sets-competitive-programming.md

@@ -31,6 +31,7 @@
 * [Exercism](http://exercism.io/)
 * [Google Code Jam - Practise](https://code.google.com/codejam/contests.html)
 * [Hacker.org](http://www.hacker.org/)
+* [Leetcode](https://leetcode.com/)
 * [Mathproblem of the Month - Bilkent University](http://www.fen.bilkent.edu.tr/~cvmath/prob-month.html)
 * [PEG Judge](http://wcipeg.com/)
 * [PKU Online Judge](http://poj.org/)