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 Bare 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.

使用栈。

Runtime: 0 ms, faster than 100.00% of C++ online submissions forMinimum Add to Make Parentheses Valid.

#include<string>
#include<iostream>
#include <stack>

using namespace std;

class Solution {
public:
    int minAddToMakeValid(string S) {
        stack<int> st;
        for(auto a: S){
            if('('==a)
                st.push(a);
            else if(!st.empty()&&st.top()=='('){
                st.pop();
            }else{
                st.push(a);
            }
        }
        return st.size();
    }
};

int main(){
    Solution solution=Solution();
    cout<<solution.minAddToMakeValid("())")<<endl; //1
    cout<<solution.minAddToMakeValid("(((")<<endl; //3
    cout<<solution.minAddToMakeValid("()")<<endl;  //0
    cout<<solution.minAddToMakeValid("()))((")<<endl; //4
    cout<<solution.minAddToMakeValid("(()())((")<<endl;//2

}