using System;

using System.Collections.Generic;                   

​

public class Program

{

    public static void Main()

    {

        int[] input = {4, 2, 1, 6, 5}; // 4,5,6 => 3

        var result = FindConsecutiveArray(input);

        Console.WriteLine("Result: " + result);

        int[] input2 = {5,5,3,1}; // 5,3,1 => 1

        var result2 = FindConsecutiveArray(input2);

        Console.WriteLine("Result: " + result2);

    }

    public static int FindConsecutiveArray(int[] array)

    {

        var map = new HashSet<int>();

        int max = int.MinValue;

        // Read all elements and store them in HashSet

        foreach(int i in array)

        {

            map.Add(i);

        }

        int tempMax = 1;

        foreach(var i in map)

        {

            // If i is not the leftmost value in the consecutive numbers, we should skip

            if(map.Contains(i-1))

                continue;

            // If i is the leftmost value, start counting

            else

            {

                var tempKey = i;

                while(map.Contains(++tempKey))

                {

                    tempMax++;

                }

                max = Math.Max(tempMax, max);

                tempMax = 1;

            }

        }

        return max;

    }   

}