Longest substring which contains just two unique characters

This was discussed at http://www.mitbbs.com/article_t/JobHunting/32444763.html

Here is my solution in (C#):

 // ----------------------------------------------------------------------------
// Copyright (c) SimpleCoder 2013
// ----------------------------------------------------------------------------
using System.Collections.Generic;
using Microsoft.VisualStudio.TestTools.UnitTesting;
namespace LongestSubstring
{
static class Program
{  
    // Find longest substring which contains just two unique characters.
    static string LongestSubstring(string str)
    {
        int Len = str.Length;
        // start index of current substring
        int i = 0;
        // end index of current substring
        int j = 0;            
        // start index of the longest substring
        int maxStart = 0;
        // length of the longest substring
        int maxLen = 0;
        // Occurrence of each char in current substring
        Dictionary<char, int> CountMap = new Dictionary<char, int>();
           
        while (j < Len) 
        {
           char ch = str[j];
           if (!CountMap.ContainsKey(ch))
               CountMap.Add(ch, 1);
           else 
               ++CountMap[ch];
           // Increment i until there are at most
           // 2 unique chars in current substring begin at i and end at j
           while (CountMap.Count > 2 && i < j)
           {
               ch = str[i];
               if (CountMap[ch] > 1) 
                   --CountMap[ch];
               else
                   CountMap.Remove(ch);                                  
               ++i;                           
           }            
           // If we got 2 chars in current substring and its length
           // is the longest so far, update maxStart and maxLen
           if (CountMap.Count == 2 && j - i >= maxLen) 
           { 
               maxStart = i; 
               maxLen = j - i + 1; 
           }                           
           ++j; 
       }
       return str.Substring(maxStart, maxLen); 
    }
    static void Main() 
    {
       Assert.AreEqual("", LongestSubstring(""));
       Assert.AreEqual("", LongestSubstring("a"));
       Assert.AreEqual("ab", LongestSubstring("ab"));
       Assert.AreEqual("cccccaaacccc", LongestSubstring("abaaabcccccaaaccccdd")); 
    } 
 }
}
 

Follow up, take a look at solution for a related but slightly different question:
Longest substring which begins and ends with two unique characters