Leetcode 76 Minimum windows substring - write C# code based on one of discussion, pass Leetcode online judge

Leetcode76_minimumWindowSubstring.cs

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

namespace Leetcode76_minimum_window_substring
{
    /// <summary>
    /// Leetcode 76: Minimum window substring
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
        }

        /// <summary>
        /// code review - Julia works on the code on April 3, 2018
        /// </summary>
        /// <param name="source"></param>
        /// <param name="pattern"></param>
        /// <returns></returns>
        public String minWindow(String source, String pattern) {
          var map = new int[256];

          foreach(char c in pattern.ToCharArray()){
            map[c - 'A']++;
          }
      
          int minLen = int.MaxValue;
          int minStart = 0;
  
          var totalNeedFound = pattern.Length;

          var sourceCharArray = source.ToCharArray();

          int start = 0;
          int end   = 0;

          while(end < sourceCharArray.Length){
            int endChar = sourceCharArray[end] - 'A';

            map[endChar]--;

            if(map[endChar] >= 0){ // still need more endChar
              totalNeedFound--; 
            }
        
            if(totalNeedFound == 0){
              int startChar = sourceCharArray[start] - 'A';

              while(map[startChar] < 0){  // extra char is found
                map[startChar]++;
                start++;
                startChar = sourceCharArray[start] - 'A';  
              }
          
              int currentLength = end - start + 1;

              if(currentLength < minLen){
                minLen   = currentLength;
                minStart = start;
              }

              map[startChar]++;
              start++;
              totalNeedFound++;
            }

            end++;
          }
  
          return minLen == int.MaxValue ? "" : source.Substring(minStart, minLen);
        }
    }    
}