Groovy Permutations

def perm(tokens) {
    if (!tokens || tokens.size() == 1) {
        return [tokens];
    }

    if (tokens.size() == 2) {
        return [tokens, tokens.reverse()];
    }

    def permutations = []
    for (def token : tokens) {
        def tail = tokens.collect() - token;
        def tail_perms = perm(tail)
        for (def p : tail_perms) {
            p.add(0, token)
            permutations += [p]
        }
    }
    return permutations
}

assert [['a', 'b'], ['b', 'a']] == perm('ab'.toList())
assert [['a', 'b', 'c'], ['a', 'c', 'b'], ['b', 'a', 'c'], ['b', 'c', 'a'], ['c', 'a', 'b'], ['c', 'b', 'a']] == perm('abc'.toList())
assert ['d', 'c', 'b', 'a'] == perm('abcd'.toList())[-1]