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.

class node{
    char c= ' ';
    int i=0;
    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> stack = new Stack<node> ();
        stack.push(new node(')',-1));
        for(int i=0;i<len;i++){
            char c = s.charAt(i);
            if(c=='('){
                stack.push(new node(c,i));
            }
            else{
                if(stack.peek().c=='('){
                    stack.pop();
                    max = Math.max(max,i-stack.peek().i);
                }
                else{
                    stack.push(new node(c,i));
                }
            }
        }
        return max;
    }
}