[leetcode-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. 

思路：

用一个栈即可，如果顶部和字符串中的当前字符匹配则出栈，最后栈的大小即为不匹配的个数。

int minAddToMakeValid(string S) {
        if(S.length()<=1)return S.length();
        stack<char>st;
        for(int i = 0; i < S.length(); i++)
        {
            if(S[i] == ')' && !st.empty() && st.top() == '(')
            {
                st.pop();
                continue;
            }
            else
            {
                st.push(S[i]);
            }
        }
        return st.size();
    }