private static bool AreBracketsValid(string input)
{
    const string OPEN = "([{<";
    const string CLOSE = ")]}>";

    bool result = true;

    Stack<char> s = new Stack<char>();

    foreach (char c in input)
    {

        // Push open characters to stack
        if (OPEN.Contains(c))
        {
            s.Push(c);
        }

        // If close character matches open character, remove open character from stack
        else if (OPEN.IndexOf(s.Peek()) == CLOSE.IndexOf(c))
        {
            s.Pop();
        }

        // No match, invalid brackets
        else
        {
            result = false;
            break;
        }
    }

    return result;
}