using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public void Main()

{

    var list = new List<List<int>>

    {

        new List<int> {2},

        new List<int> {2,2,2},

        new List<int> {1,1,2,2,2,3,3,3,3},

        new List<int> {5,4,5,4,3,5,4,5,4,5,4,3,5,4,5,3,3,3,4,5,4,5,4,5,4,3,3,3,5,4},

        new List<int> {2,2,11,11,11,11,6,2,2,6,6,2,2,2,2,6,11,6,11,11,2,11,2,6,6,11,2,11,6,2,6,6,6,6,6,11,2,11,11,6,6,6,11,6,2,6,2,6,6,2,11,2,2,11,11,11,11,2,6,2}

    };

    var refList = new List<List<int>>

    {

        new List<int> {2},

        new List<int> {2,2},

        new List<int>{2,2,3,3,3},

        new List<int>{5,4,5,4,3,5,4,5,4,3,5,4,5,3,3,4,5,5,4,4,3,3,5,4},

        new List<int> {2,11,11,11,11,6,2,6,6,2,2,6,11,6,11,11,2,11,6,11,2,11,6,6,6,6,6,2,11,11,6,6,6,11,6,2,6,6,11,2,11,11,11,11,2,6}

    };

    for(int i = 0; i < list.Count; i++)

    {

        R(list[i]).SequenceEqual(refList[i]).Dump();

    }

}

​

// The method

List<int>R(List<int>l,int n=1)

{for(int i=0,t=0;i<l.Count;i++)

{if(l[i]!=n)continue;t++;if(t==n)

{l[i]=0;t=0;}}

return n>l.Max()?l:R(l.Where(w=>w!=0).ToList(),++n);}

}