using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {    // findLongestString("abcd", 2);

        // findLongestString("aabcbb", 2);

        // findLongestString("abbcb", 1);

        // findLongestString("abccde", 1);

        // findLongestString("aabcdbb", 2);

    }

​

    private static void findLongestString(string actualString, int allowedReplacement)

    {

        int WindowStart = 0;

        int WindowEnd = 0;

        Dictionary<char, int> uniqueBucket = new Dictionary<char, int>();

        int maxRepetitiveCharacter = 0;

        string LongestString = string.Empty;

        int LongestStringLength = 0;

        for (WindowEnd = 0; WindowEnd < actualString.Length; WindowEnd++)

        {

            // First check in given window max repetitive character and then check if in that window number of character other than max repetitive character is more than allowedReplacement then change window.

            // Add character to Dictionary.

            char character = actualString[WindowEnd];

            if (!uniqueBucket.ContainsKey(character))

            {

                uniqueBucket.Add(character, 1);

            }

            else

            {

                uniqueBucket[character]++;

            }

            maxRepetitiveCharacter = Math.Max(maxRepetitiveCharacter, uniqueBucket[character]);

            // Check for number of character other than max repetitive character. if greater than allowedReplacement then change window.

            while (WindowEnd - WindowStart + 1 - maxRepetitiveCharacter > allowedReplacement)

            {       

                LongestString = actualString.Substring(WindowStart, (WindowEnd - WindowStart));

                LongestStringLength = Math.Max(LongestStringLength, LongestString.Length);

                Console.WriteLine("String '" + actualString.Substring(WindowStart, (WindowEnd - WindowStart)) + "' has length " + LongestString.Length);

                char leftCharacter = actualString[WindowStart];

                maxRepetitiveCharacter = 1;

                uniqueBucket[leftCharacter]--;

                WindowStart++;              

            }

​

            if (WindowEnd == actualString.Length - 1 && !(WindowEnd - WindowStart + 1 - maxRepetitiveCharacter > allowedReplacement))

            {

                LongestString = actualString.Substring(WindowStart, (WindowEnd - WindowStart + 1));

                LongestStringLength = Math.Max(LongestStringLength, LongestString.Length);

                Console.WriteLine("String '" + actualString.Substring(WindowStart, (WindowEnd + 1 - WindowStart)) + "' has length " + LongestString.Length);

            }

        }

​

        Console.WriteLine("Longest substring length with same character '" + LongestStringLength);

    }

}