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