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

First of all, sets have permutations, not strings or integers, so I'll just assume you mean "the set of characters in a string."

Note that a set of size n has n! n-permutations.

The following pseudocode (from Wikipedia), called with k = 1...n! will give all the permutations:

function permutation(k, s) {
    for j = 2 to length(s) {
        swap s[(k mod j) + 1] with s[j]; // note that our array is indexed starting at 1
        k := k / j; // integer division cuts off the remainder
    }
    return s;
}

Here's the equivalent Python code (for 0-based array indexes):

def permutation(k, s):
    r = s[:]
    for j in range(2, len(s)+1):
        r[j-1], r[k%j] = r[k%j], r[j-1]
        k = k/j+1
    return r