Listing all permutations of a string/integer

Question

A common task in programming interviews (not from my experience of interviews though) is to take a string or an integer and list every possible permutation.

Is there

Answer 1

Building on @Peter's solution, here's a version that declares a simple LINQ-style Permutations() extension method that works on any IEnumerable.

Usage (on string characters example):

foreach (var permutation in "abc".Permutations())
{
    Console.WriteLine(string.Join(", ", permutation));
}

Outputs:

a, b, c
a, c, b
b, a, c
b, c, a
c, b, a
c, a, b

Or on any other collection type:

foreach (var permutation in (new[] { "Apples", "Oranges", "Pears"}).Permutations())
{
    Console.WriteLine(string.Join(", ", permutation));
}

Outputs:

Apples, Oranges, Pears
Apples, Pears, Oranges
Oranges, Apples, Pears
Oranges, Pears, Apples
Pears, Oranges, Apples
Pears, Apples, Oranges

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

public static class PermutationExtension
{
    public static IEnumerable Permutations(this IEnumerable source)
    {
        var sourceArray = source.ToArray();
        var results = new List();
        Permute(sourceArray, 0, sourceArray.Length - 1, results);
        return results;
    }

    private static void Swap(ref T a, ref T b)
    {
        T tmp = a;
        a = b;
        b = tmp;
    }

    private static void Permute(T[] elements, int recursionDepth, int maxDepth, ICollection results)
    {
        if (recursionDepth == maxDepth)
        {
            results.Add(elements.ToArray());
            return;
        }

        for (var i = recursionDepth; i <= maxDepth; i++)
        {
            Swap(ref elements[recursionDepth], ref elements[i]);
            Permute(elements, recursionDepth + 1, maxDepth, results);
            Swap(ref elements[recursionDepth], ref elements[i]);
        }
    }
}