ACM
VJ https://vjudge.net/
AOJ http://judge.u-aizu.ac.jp/onlinejudge/
gcj http://code.google.com/codejam
ccpc http://ccpc.io/
poj http://poj.org/
hdu http://acm.hdu.edu.cn/
cf http://codeforces.com/
51nod http://www.51nod.com/focus.html
hihocoder http://hihocoder.com/hiho
topcoder https://www.topcoder.com/
计蒜客 https://nanti.jisuanke.com/
牛客网 https://www.nowcoder.com/
poj题库 很好很有层次感 http://blog.csdn.net/a1dark/article/details/11714009
信息站 http://acmicpc.info/
https://icpcarchive.ecs.baylor.edu/index.php
leetcode http://www.lintcode.com/zh-cn/
pat https://www.patest.cn/p/index#test