(字符串 栈) leetcode 921. Minimum Add to Make Parentheses Valid

Given a string S of '(' and ')' parentheses, we add the minimum number of parentheses ( '(' or ')', and in any positions ) so that the resulting parentheses string is valid.

Formally, a parentheses string is valid if and only if:

• It is the empty string, or
• It can be written as AB (A concatenated with B), where A and Bare valid strings, or
• It can be written as (A), where A is a valid string.

Given a parentheses string, return the minimum number of parentheses we must add to make the resulting string valid.

Example 1:

Input: "())"
Output: 1

Example 2:

Input: "((("
Output: 3

Example 3:

Input: "()"
Output: 0

Example 4:

Input: "()))(("
Output: 4

Note:

1. S.length <= 1000
2. S only consists of '(' and ')' characters.

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这个题是栈的基础题，不过可以用O(n)的时间复杂度和O(1)的空间复杂度来解这个问题。参考题解：https://leetcode.com/articles/minimum-add-to-make-parentheses-valid/

C++代码：

class Solution {
public:
    int minAddToMakeValid(string S) {
        int ans = 0,bal = 0;
        for(int i = 0; i < S.size(); i++){
            bal += S[i] == '('?1:-1;
            if(bal == -1){
                ans++;
                bal++;
            }
        }
        return ans+bal;
    }
};