Unique permutations of a list of strings

UniquePermutations.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PermutationsUnqiue
{
  class Program
  {
    static void Main(string[] args)
    {
      List<String> str = new List<string>() { "hello", "world", "hello" };
      List<string> results = new List<string>();
      int n = str.Count;
      Permute(str, 0, n - 1, ref results);
      Console.WriteLine("yeet");
      foreach (string s in results) {
        Console.WriteLine(s);
      }
      Console.ReadKey();
    }

    #region permutations
    private static bool ShouldSwap(List<string> phr, int start, int curr)
    {
      for (int i = start; i < curr; i++) {
        if (phr[i] == phr[curr]) {
          return false;
        }
      }
      return true;
    }

    private static List<string> Permutations(List<string> phrase)
    {
      List<string> results = new List<string>();
      int n = phrase.Count;
      Permute(phrase, 0, n - 1, ref results);
      return results;
    }

    private static void Permute(List<string> phrase, int l, int r, ref List<string> results)
    {
      if (l == r) {
        results.Add(String.Join(" ", phrase.ToArray()));
      }
      else {
        for (int i = l; i <= r; i++) {
          bool check = ShouldSwap(phrase, l, i);
          if (check) {
            phrase = Swap(phrase, l, i);
            Permute(phrase, l + 1, r, ref results);
            phrase = Swap(phrase, l, i);
          }
        }
      }
    }

    private static List<string> Swap(List<String> words, int i, int j)
    {
      string temp;
      temp = words[i];
      words[i] = words[j];
      words[j] = temp;
      return words;
    }
    #endregion 
  }
}