Merge branch 'pr/1971'

problem-sets-competitive-programming.md

@@ -49,6 +49,7 @@
 * [Google Code Jam - Practise](https://code.google.com/codejam/contests.html)
 * [Hacker.org](http://www.hacker.org)
 * [HackerEarth](https://www.hackerearth.com)
+* [HDU Online Judge](http://acm.hdu.edu.cn)
 * [InterviewBit](https://www.interviewbit.com)
 * [Leetcode](https://leetcode.com)
 * [Mathproblem of the Month - Bilkent University](http://www.fen.bilkent.edu.tr/~cvmath/prob-month.html)