ExPermutations.cs

ExPermutations.cs

using System;
using System.Linq;
using System.Collections.Generic;

static class Ex {

  public static IEnumerable<IEnumerable<T>> RotationsL<T>(this IEnumerable<T> xs) {
    for(int i = xs.Count(); i > 0; i--) {
      yield return xs;
      xs = xs.Concat(xs.Take(1)).Skip(1);
    }
  }

  public static IEnumerable<IEnumerable<T>> Permutations<T>(this IEnumerable<T> xs) {
    if(xs.Count() == 0) yield return Enumerable.Empty<T>();
    foreach(var ys in xs.RotationsL()) {
      var zs = ys.Take(1);
      foreach(var ws in ys.Skip(1).Permutations()) {
        yield return zs.Concat(ws);
      }
    }
  }
}

// Examples

class Program {
  
  public static void Main(string[] args) {

    foreach(var xs in Enumerable.Range(1,3).Permutations()) {
      Console.WriteLine(string.Join("-", xs.Select(x=>x.ToString()).ToArray()));
    }

    foreach(var xs in "ABCD".Permutations()) {
      Console.WriteLine(string.Join("-", xs.Select(x=>x.ToString()).ToArray()));
    }
  }
}