LeetCode 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.

public class Solution {
    public int longestValidParentheses(String s) {
        if (s == null) {
            return  0;
        }
        int maxlen=0;
        Stack<Integer> stack = new Stack<Integer>();
        stack.add(-1);
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                stack.add(i);
            } else {
                stack.pop();
                if (stack.isEmpty()) {
                    stack.add(i);
                } else {
                    maxlen=Math.max(maxlen,i-stack.peek());
                } 
            }
        }
        return maxlen;
    }
}