leetcode讲解--921. Minimum Add to Make Parentheses Valid

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:

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

题目地址

这是栈的经典应用

java代码

class Solution {
    public int minAddToMakeValid(String S) {
        Stack<Character> stack = new Stack<>();
        char[] c = S.toCharArray();
        for(int i=0;i<c.length;i++){
            if(c[i]==')' && !stack.empty() && stack.peek()=='('){
                stack.pop();
            }else{
                stack.push(c[i]);
            }
        }
        return stack.size();
    }
}