using System;

public class Program

{

    public static void Main()

    {

        var arr = new int[] { 1,2,3,4,4,4,7,7,12,12,13 };

​

        Console.WriteLine(countUniqueValues(arr)); // 7

    }

    public static int countUniqueValues(int[] arr){

        if(arr.Length == 0){

            return 0;

        }

        var i = 0;

        for(var j=1; j < arr.Length; j++){

            if(arr[i] != arr[j]){

                i++;

                arr[i] = arr[j];

            }

        }

        return i + 1;

    }

}