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 int longestValidParentheses(String s) {
    	if(s==null||s.length()==0){
    		return 0;
    	}
    	Stack
  
    stack = new Stack
   
    (); int lastposition = -1;// the last not match position of ")" int res = 0; for(int i =0;i