[LeetCode 32] Longest Valid Parentheses

题目

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

测试案例

Input: "(()"Output: 2Explanation: The longest valid parentheses substring is "()"Input: ")()())"Output: 4Explanation: The longest valid parentheses substring is "()()"

思路

1. 采用栈数据结构。栈中存放各字符的下标。初始时里面放入-1。

2. 从左至右依次遍历每个字符。当前字符为左括号就进栈。当前字符为右括号时，如果栈中存在左括号，则出栈。否则，入栈。
3. 每当都元素，记下标为 i ，进栈时，就用 i - stack.peek() - 1 更新 max。

4. 遍历结束后，需要用 n - stack.peek() - 1 更新 max。

代码如下

class Solution {    public int longestValidParentheses(String s) {                int max = 0, n = s.length(), temp, index = 0;         if(n == 0){            return 0;        }        int[] stack = new int[n + 1];                stack[index++] = -1;        for(int i = 0; i < n; i++){            if(s.charAt(i) == '(' || (temp = stack[index - 1]) == -1 ||                s.charAt(temp) == ')'){                                               if((temp = i - stack[index - 1] - 1) > max){                    max = temp;                }                                stack[index++] = i;            }            else{                index--;                            }                    }        if((temp = n - stack[index - 1] - 1) > max){            max = temp;        }                return max;    }}