Leetcode Longest Valid Parentheses

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.

对于这道题，用动态规划方法时间超时，后来看了别人的解法

建立一个栈，用来保存（的位置信息，同时用一个符号表示第一个（的位置信息，如果遇到（就压站，如果遇到）且栈不为空的时候就弹出，并判断弹出后是否为空，若为空则最大长度为i-左括号起始位置，若不为空则减去peek的位置

package Longest.Valid.Parentheses;

import java.util.Stack;

public class LongestValidParentheses {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        LongestValidParentheses service=new LongestValidParentheses();
        //String s=")()())";
        String s="()(()";
        //String s="()";
        int a=service.longestValidParentheses(s);
        System.out.println(a);
    }
    public int longestValidParentheses(String s) {
        int len=s.length();
        Stack<Integer> stack=new Stack<Integer>();
        char[] charArray=s.toCharArray();
        int maxCount=0;
        int firstleft=-1;
        for(int i=0;i<len;i++){
            if(charArray[i]=='(')
            {
                stack.push(i);
            }else{
                if(!stack.isEmpty()){
                stack.pop();
                if(stack.isEmpty()){
                    maxCount=Math.max(maxCount, i-firstleft);
                }else{
                    maxCount=Math.max(maxCount, i-stack.peek());
                }
                }else{
                    firstleft=i;
                }
            }
        }
        return maxCount;
    }
}