LeetCode 921

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.

解题思路：遍历字符串，遇到‘(’和')' 存放到栈里，只要有配对的就从栈中弹出，最后栈中剩下的数量就是所求值。

class Solution {
public:
    
    int minAddToMakeValid(string S) {
        stack<char> myStack;
        for (auto c : S) {
            if (c == '(') {
                myStack.push(c);
            } else if (c == ')') {
                if (myStack.size() > 0) {
                    if (myStack.top() == '(') {
                        myStack.pop();
                    } else {
                        myStack.push(c);
                    }
                } else {
                    myStack.push(c);
                }
            }
        }
        return myStack.size();
    }
};