public void SortString_1()
        {
            var solution = new _1370_IncreasingDecreasingString();
            var result   = solution.SortString("aaaabbbbcccc");

            Assert.AreEqual("abccbaabccba", result);
        }
        public void SortString_3()
        {
            var solution = new _1370_IncreasingDecreasingString();
            var result   = solution.SortString("leetcode");

            Assert.AreEqual("cdelotee", result);
        }
        public void SortString_5()
        {
            var solution = new _1370_IncreasingDecreasingString();
            var result   = solution.SortString("spo");

            Assert.AreEqual("ops", result);
        }
        public void SortString_2()
        {
            var solution = new _1370_IncreasingDecreasingString();
            var result   = solution.SortString("rat");

            Assert.AreEqual("art", result);
        }