【练习】括号类问题汇总

20. 有效的括号 https://leetcode.cn/problems/valid-parentheses

921. 使括号有效的最少添加 https://leetcode.cn/problems/minimum-add-to-make-parentheses-valid

1541. 平衡括号字符串的最少插入次数 https://leetcode.cn/problems/minimum-insertions-to-balance-a-parentheses-string