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.

 维护一个栈，最初栈添加一个Node(')', -1)。

if (s.charAt(i) == ')'  )

　　if (&& 栈顶为 '(')

　　'(' 出栈 计算长度 Math.max( max,  i - stack.peek().下标)

　　计算长度时是当前下标，和'(' 前一个元素下标的差值；

　　else 

　　　　')' 入栈

else

　　'(' 入栈

class Node{
    int i;
    char c;
   public Node(char c, int i){
       this.i = i;
       this.c = c;
   }
}
public class Solution {
    public int longestValidParentheses(String s) {
        int len = s.length();
        int max = 0;
        Stack<Node> nStack = new Stack<Node>();
        nStack.push(new Node(')', -1));
        for(int i = 0; i< len; i++){
            char c = s.charAt(i);
            if(c == '('){
                nStack.push(new Node(c, i));
            }else{
                Node n = nStack.peek();
                if(n.c == '('){
                    nStack.pop();
                    max = Math.max(max, i - nStack.peek().i);
                }else{
                    nStack.push(new Node(c,i));
                }
            }
        }
        
        return max;
    }
}