HackerRank - String - Reverse Shuffle Merge - Third practice in a row - work on issues.

reverseShuffleMerge_HackerRank_PracticeII.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace reverseShuffleMerge_Practice3
{
    class Program
    {
        static void Main(string[] args)
        {
            //runTestCases(); 
            Console.WriteLine(reverseShuffleMerge(Console.ReadLine())); 
        }

        public static void runTestCases()
        {
            string[] testCases = new string[4] { "abcacb", "aaccbb", "cacbab", "baabcc" };

            List<string> outputStrs = new List<string>();
            foreach (string s in testCases)
            {
                string s2 = reverseShuffleMerge(s);

                outputStrs.Add(s2);
            }

            string[] result = outputStrs.ToArray(); 
        }

        public static string reverseShuffleMerge(string input)
        {
            if (input == null || input.Length == 0)
                return null;

            int len = input.Length;
            int TOTALCHARS = 26;

            int[] moreToAdd   = new int[TOTALCHARS];
            int[] moreToSkip  = new int[TOTALCHARS];

            foreach (char c in input)
            {
                moreToAdd[c - 'a']++; 
            }

            for (int i = 0; i < TOTALCHARS; i++)
            {
                moreToAdd[i]   = moreToAdd[i] / 2;
                moreToSkip[i]  = moreToAdd[i]; 
            }

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

            for (int i = len - 1; i >= 0; i--)
            {
                char runner = input[i];
                int  index  = runner - 'a'; 

                while (stack.Count > 0             &&
                       moreToAdd[index] > 0        &&                       
                       (char)stack.Peek() > runner &&
                       moreToSkip[(char)stack.Peek() - 'a'] > 0                       
                    )
                {
                    // "abcacb", c is on top of stack, 
                    // 'c' - 'a' = 2, 
                    // moreToAdd[2] = 0, moreToSkip[2] = 1; 
                    // pop out 'c', and then, adjust the value for 'c'
                    // moreToAdd[2] = 1, and moreToSkip[2] = 0.
                    // while will be excuted two times, one for 'c', one for 'b'
                    char backTracked = (char)stack.Pop();  // bug: do not overwrite runner

                    int oldIndex = backTracked - 'a';

                    moreToAdd[oldIndex]++;
                    moreToSkip[oldIndex]--; 
                }

                if (moreToAdd[index] > 0)
                {
                    stack.Push(runner);
                    moreToAdd[index]--;
                }
                else
                {
                    moreToSkip[index]--; 
                }                    
            }

            char[] arr = stack.ToArray();
            Array.Reverse(arr);

            return new string(arr);             
        }
    }
}