题解 | 最长的括号子串

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param s string字符串 
     * @return int整型
     */
    public int longestValidParentheses (String s) {
        // write code here
        if(s == null || s.length() == 0) return 0;
        Stack<Integer> stack = new Stack<>();
        //压入-1作为哨兵，1 - (-1) = 2
        stack.push(-1); 
        int maxLength = 0;
        for(int i = 0;i<s.length();i++){
            int c = s.charAt(i);
            if(c == '(')
            stack.push(i);
            else
            stack.pop();

            if(stack.isEmpty()){
                stack.push(i);//作为新边界
            }
            else
            maxLength = Math.max(maxLength, i - stack.peek());
        }
        return maxLength;
    } 
}