multiple sites:
- www.leetcode.com
- www.leetcode-cn.com
- www.leetcode.cn
- https://binarysearch.com/
- https://adventofcode.com
- https://foobar.withgoogle.com/
- https://www.lintcode.com/
- https://www.luogu.com.cn/
- https://www.nowcoder.com/
- https://www.patest.cn/
- http://poj.org
- https://www.pramp.com/dashboard#/
- http://www.pythonchallenge.com/
- https://www.topcoder.com
- https://codeforces.com/
- https://neetcode.io/ # will jump to leetcode.com
- multiple solutions including attempts and various optimization
- multiple languages: Python2/Python3/C/CPP/Java/Golang/CSharp/JavaScript/Bash shell/Rust/SQL