public class Solution
{
    struct Item
    {
        public char Sym;
        public int Count;
        public char? PrevSym;
    }

    public int LongestValidParentheses(string s)
    {
        var stack = new Stack<Item>();
        var answer = 0;
        foreach (var ch in s)
        {
            if (stack.Count > 0 && (stack.Peek().Sym == '(' || stack.Peek().PrevSym == '(') && ch == ')')
            {
                var prev = stack.Pop();
                var count = 2;
                if (prev.Sym == '#')
                {
                    stack.Pop();
                    count += prev.Count;
                }
                while (stack.Count > 0 && stack.Peek().Sym == '#')
                    count += stack.Pop().Count;
                char? prevSym = null;
                if (stack.Count > 0)
                    prevSym = stack.Peek().Sym;
                stack.Push(new Item() { Sym = '#', Count = count, PrevSym = prevSym });
                answer = Math.Max(answer, count);
            }
            else
            {
                stack.Push(new Item() { Sym = ch, Count = 0 });


            }
        }

        return answer;
    }
}