using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {   

        /*

        Assumptions:

        - Input may be invalid and so the parameters should be validated.

        - An exception should be thrown in case of invalid parameters.

        - The code is finding the last pattern with the highest number of occurances in the 

        string. Eg: if a string is composed of aabbaabb and the pattern length is 2, bb : 2

        will be returned, as that is the latest pattern.

        - Pattern occurances can overlap. Eg: asasa with a pattern length of 3 will return

        asa : 2.

​

        */

        Print(PatternOccurances("zfabcde224lkfabc51+crsdtab=", 3));

        Print(PatternOccurances("abc++---+ssa---", 2));

        Print(PatternOccurances("zfabcde224lkfabc51+crsdtab=", 3));     

    }

    public static KeyValuePair<string,int> PatternOccurances(string s, int patternLength)

    {

        if (string.IsNullOrEmpty(s) || patternLength <= 0 || patternLength > s.Length)

            throw new ArgumentException("Invalid parameters!");

        Dictionary<string, int> patternOccurances = new Dictionary<string, int>();

​

        for (int i = 0; i < s.Length - patternLength + 1; i++)

        {

            string pattern = s.Substring(i, patternLength);

            if (patternOccurances.ContainsKey(pattern))

                patternOccurances[pattern]++;

            else

                patternOccurances[pattern] = 1;

        }

        // We could have kept track of the final pattern with the highest occurance count

        // in the previous loop and simply returned it here, however I believe the code

        // is more maintainable using LINQ and the performance impact is negligible.

        int maxOccurances = patternOccurances.Values.Max();

        return patternOccurances.Last(x => x.Value == maxOccurances);

    }

    public static void Print(KeyValuePair<string,int> result)

    {

        Console.WriteLine("Pattern: \"" + result.Key + "\" Occurance: " + result.Value);    

    }

}