Leetcode Longest Valid Parentheses 结题报告

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

For "(()", the longest valid parentheses substring is "()", which has length = 2.

Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

https://oj.leetcode.com/problems/longest-valid-parentheses/

分析，求出一串由：‘(’和‘)’组成的字符串中合法()配对的长度。例如：(()()，结果是4。((()))结果是6。())()()结果是4。

解题思路：最容易想到的解法就是穷举法，计算任意两点(i到j)之间有多少个合法的()串，借助动态规划可以得到结果。算法复杂度为：

想要O(n)的解法需要一点技巧，也要弄清楚几种情况：

AC代码如下所示：

public class Solution {
    public int longestValidParentheses(String s) {
        if(s==null||s.length()==0) {
            return 0;
        }
        int start     = -1;
        int maxLength = 0;
        Stack stack   = new Stack();
        for(int i=0;i<s.length();i++) {
            if(s.charAt(i)=='(') {
                stack.push(i);
            } else {
                if(!stack.empty()) {
                    stack.pop();
                    if(stack.empty()==true) {
                        maxLength = Math.max(i - start , maxLength);
                    } else {
                        maxLength = Math.max(i - (int)stack.peek() , maxLength);
                    }
                } else {
                    start = i;
                }
            }
        }

        return maxLength;
    }
}