leetcode 32. Longest Valid Parentheses——（use stack）

import java.util.Stack;

//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.

public class Solution {

public static void main(String[] args) {
String input = "(())()";
int output = longestValidParentheses(input);
System.out.println(output);
}

public static int longestValidParentheses(String s) {
if(s.length() == 0){
return 0;
}
Stack<Integer> stack = new Stack<Integer>();
int lengthBefore = 0;														//为括号之前有多少已经匹配成功的括号串
int max = 0;
if(s.charAt(0) == '('){
stack.push(lengthBefore);
}
for(int i = 1;i<s.length();i++){
if(s.charAt(i) == '('){
stack.push(lengthBefore);											//将'('前已经成功匹配的有效括号串push入栈中
lengthBefore = 0;
}else{
if(stack.isEmpty()){
lengthBefore = 0;
}else{
lengthBefore = lengthBefore+stack.pop()+2;						//')'成功匹配后，有效括号串的长度为：2+匹配成功的'('之前的有效括号串长度+本反括号')'之前的有效括号串长度
}
max = Math.max(max, lengthBefore);
}
}
return max;
}

}