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

Note:

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

Approach #1: C++.[stack]

class Solution {
public:
    int minAddToMakeValid(string S) {
        int len = S.length();
        if (len == 0) return 0;
        stack<char> myStack;
        myStack.push(S[0]);
        
        for (int i = 1; i < len; ++i) {
            if (myStack.empty()) myStack.push(S[i]);
            else if (myStack.top() == '(' && S[i] == ')') myStack.pop();
            else myStack.push(S[i]);
        }
        
        return myStack.size();
    }
};