全排列（permutation)

def (foo):
    if foo is None:
        return

    arr = list(foo)
    def allRange(arr, start = 0, length = len(arr)):
        if start is length - 1:
            print('' . join(arr))
        for i in range(start, length):
            [arr[start], arr[i]] = [arr[i], arr[start]]
            allRange(arr, start + 1)
            [arr[start], arr[i]] = [arr[i], arr[start]]

    allRange(arr)