Longest Valid Parentheses

用stack，并记录入栈的括号的位置

public class Solution {
    class Node{
        char c;
        int nub;
        public Node(char c, int nub){
            this.c = c;
            this.nub = nub;
        }
    }
    
    public int longestValidParentheses(String s) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        
        char[] mychar = s.toCharArray();
        int result = 0;
        int tmp = 0;
        Stack<Node> mystack = new Stack<Node>();
        mystack.push(new Node(')', -1));
        
        for(int i=0; i < mychar.length; ++i)
        {
            if(mychar[i] == '(')
                mystack.push(new Node('(',i));
            else
            {
                if(mystack.peek().c == '(')
                {
                    mystack.pop();
                    result = Math.max(result, i - mystack.peek().nub);
                }
                else
                {
                     mystack.push(new Node(')',i));
                }
            }
        }
        return result;
    }
}