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

Base/Revise on Pengyang answer

And inspired from permutations-in-javascript

The c# version FunctionalPermutations should be this

static IEnumerable> FunctionalPermutations(IEnumerable elements, int length)
    {
        if (length < 2) return elements.Select(t => new T[] { t });
        /* Pengyang answser..
          return _recur_(list, length - 1).SelectMany(t => list.Where(e => !t.Contains(e)),(t1, t2) => t1.Concat(new T[] { t2 }));
        */
        return elements.SelectMany((element_i, i) => 
          FunctionalPermutations(elements.Take(i).Concat(elements.Skip(i + 1)), length - 1)
            .Select(sub_ei => new[] { element_i }.Concat(sub_ei)));
    }