順列をC#で

Program.cs

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

class Program
{
    static void Main(string[] args)
    {
        var ps = "0123".Permute(3);
        Console.WriteLine(string.Join(",", ps.Select(x => new string(x.ToArray()))));
    }
}

public static class EnumerableEx
{
    public static IEnumerable<IEnumerable<T>> Dist<T>(this IEnumerable<T> source, T element)
    {
        if (source.Any())
        {
            var head = source.First();
            var tail = source.Skip(1);
            yield return Enumerable.Repeat(element, 1).Concat(source);
            foreach (var source2 in tail.Dist(element))
                yield return Enumerable.Repeat(head, 1).Concat(source2);
        }
        else
        {
            yield return Enumerable.Repeat(element, 1);
        }
    }

    public static IEnumerable<IEnumerable<T>> Permute<T>(this IEnumerable<T> source)
    {
        if (source.Any())
        {
            var head = source.First();
            var tail = source.Skip(1);
            foreach (var source2 in tail.Permute())
                foreach (var source3 in source2.Dist(head))
                    yield return source3;
        }
        else
        {
            yield return Enumerable.Empty<T>();
        }
    }

    public static IEnumerable<IEnumerable<T>> Permute<T>(this IEnumerable<T> source, int c)
    {
        if (c == 0)
        {
            yield return Enumerable.Empty<T>();
        }
        else if (source.Any())
        {
            var head = source.First();
            var tail = source.Skip(1);
            foreach (var source2 in tail.Permute(c))
                yield return source2;
            foreach (var source2 in tail.Permute(c - 1))
                foreach (var source3 in source2.Dist(head))
                    yield return source3;
        }
    }
}