921. Minimum Add to Make Parentheses Valid

921. Minimum Add to Make Parentheses Valid (Medium)

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.

 

Companies:
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Related Topics:
Stack, Greedy

Solution 1.

// OJ: https://leetcode.com/problems/minimum-add-to-make-parentheses-valid/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    int minAddToMakeValid(string S) {
        int left = 0, invalidRight = 0;
        for (char c : S) {
            if (c == '(') left++;
            else if (!left) invalidRight++;
            else --left;
        }
        return left + invalidRight;
    }
};