HackerRank - String - Reverse Shuffle Merge - a practice - take more than 30 minutes to write, debug, look up internet, run on HackerRank.

reverseShuffleMerge_HackerRank_Practice.cs

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

namespace reverseShuffleMerge_Practice2
{
    class Program
    {
        static void Main(string[] args)
        {

            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(); 
        }

        /*
         * practice on May 26, 2016
         * Need to walk through the code, and examine the code 
         * 
         * Look up the C# string - how to reverse a string 
         *  http://stackoverflow.com/questions/228038/best-way-to-reverse-a-string
         */
        public static string reverseShuffleMerge(string input)
        {
            if (input == null || input.Length == 0)
                return null;

            int TOTALCHARS = 26;

            int[] add = new int[TOTALCHARS];
            int[] skip = new int[TOTALCHARS];            

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

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

            Stack<char> stack = new Stack<char>(); 
            int len = input.Length;

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

                // stack keeps popping out until it is ready for runner                
                while (stack.Count > 0 && 
                       add[index]  > 0 && 
                       (  (char)stack.Peek() > runner)         && 
                          (skip[(char)stack.Peek() - 'a'] > 0)    )
                {
                    char c = stack.Pop(); 
                    add[c-'a']  ++;
                    skip[c - 'a']--;    // bug: failed test case: "abcacb", output: ab.  Should be --
                }

                if (add[index] > 0) 
                {
                    stack.Push(runner);
                    add[index]--;
                }
                else
                    skip[index]--;    // bug - skip[] - how many left to be skipped
            }

            
            char[] output = stack.ToArray(); 

            Array.Reverse(output);

            return new string(output); 
        }
    }
}