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 B are 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

class Solution {
    public int minAddToMakeValid(String S) {
        int res = S.length();
        int left = 0, right = 0;

        for(char c : S.toCharArray()) {
            if(c == '(') {
                left++;
            }
            else {
                if(left > 0) {
                    left--;
                    res-=2;
                }
            }
        }
        return res;
    }
}

右括号要匹配一定要有左括号在