public void test_V2_empty()
        {
            var validClass = new valid_parentheses();

            var s   = "";
            var res = validClass.IsValid_V2(s);

            Assert.True(res);

            s   = null;
            res = validClass.IsValid_V2(s);
            Assert.True(res);
        }
        public void test_V2_success()
        {
            var validClass = new valid_parentheses();

            string s   = "()";
            var    res = validClass.IsValid_V2(s);

            Assert.True(res);

            s   = "()[]{}";
            res = validClass.IsValid_V2(s);
            Assert.True(res);
        }
        public void test_V2_false()
        {
            var validClass = new valid_parentheses();

            var s   = "(]";
            var res = validClass.IsValid_V2(s);

            Assert.False(res);

            s   = "(]";
            res = validClass.IsValid_V2(s);
            Assert.False(res);

            s   = "]";
            res = validClass.IsValid_V2(s);
            Assert.False(res);
        }