LeetCode 921. Minimum Add to Make Parentheses Valid

原题链接在这里：https://leetcode.com/problems/minimum-add-to-make-parentheses-valid/

题目：

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

Note:

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

题解：

When encountering (, leftCount++.

When encountering ), and leftCount > 0, leftCount--.

Otherwise, that means there is ) without (, res++.

Eventually, if leftCount > 0, that means there is ( without ). res += leftCount.

Time Complexity: O(n). n = S.length().

Space: O(1).

AC Java:

class Solution {
    public int minAddToMakeValid(String S) {
        if(S == null || S.length() == 0){
            return 0;
        }
        
        int res = 0;
        int leftCount = 0;
        for(int i = 0; i<S.length(); i++){
            char c = S.charAt(i);
            if(c == '('){
                leftCount++;
            }else if(leftCount > 0){
                leftCount--;
            }else{
                res++;
            }
        }
        
        res += leftCount;
        return res;
    }
}

类似Remove Invalid Parentheses, Minimum Remove to Make Valid Parentheses.