今天店面也考了这道
Given a String str remove minimal number of "(" and ")" to make the string valid.
For example:
Given string "(a))" , you should output "(a)"
其实是 https://leetcode.com/problems/remove-invalid-parentheses/ 简化版
为什么bfs不让过？
bfs不是最优解，runtime 上比dfs差好多