OJ学习之路

ACM竞赛之输入输出：http://www.cnblogs.com/jiangxinnju/p/8506089.html
awesome-algorithm: https://github.com/apachecn/awesome-algorithm
LeetCode Online Judge: https://leetcode.com/https://leetcode-cn.com/
洛谷:https://www.luogu.org
Vijos: https://vijos.org/
Virtual Judge: https://vjudge.net/
牛客: https://www.nowcoder.com/
hihocoder: http://hihocoder.com/
cometoj: https://www.cometoj.com/
LibreOJ: https://loj.ac/
Universal Online Judge: http://uoj.ac/
青藤OJ: http://code.qingtengbc.com/
SPOJ (Sphere Online Judge): http://www.spoj.com/
OpenJudge: http://openjudge.cn/
codechef: https://www.codechef.com/
UVa Online Judge: https://uva.onlinejudge.org/
codeforces: http://codeforces.com/
AtCoder: https://atcoder.jp/
小米OJ: https://code.mi.com/site/index

高校OJ

PKU JudgeOnline: http://poj.org/
ZOJ: http://acm.zju.edu.cn/onlinejudge/
HDU Online Judge System: http://acm.hdu.edu.cn/

已经废弃的OJ

topcoder: https://www.topcoder.com/
codevs: http://codevs.cn/
猿圈: http://www.oxcoder.com/index.htm
九度OJ: http://ac.jobdu.com/
九度OJ永久关闭声明: http://www.cskaoyan.com/thread-647811-1-1.html
ACdream Online Judge: http://acdream.info/