排列

def perm(arr, stack):
    if not arr:
        print(stack)  # 到树的最后，输出结果
        tmp = stack.copy()
        res.append(tmp)

    else:  # 没有到树的叶子节点的时候，使用递归继续往下找。
        for i in range(len(arr)):
            stack.append(arr[i])
            del arr[i]
            perm(arr, stack)
            arr.insert(i, stack.pop())


arr = [1, 2, 3]
stack = []
res = []
perm(arr, stack)
print(res)


def str_sort(s=''):
    if len(s) <= 1:
        return [s]
    str_list = []
    for i in range(len(s)):
        for j in str_sort(s[0:i] + s[i + 1:]):
            str_list.append(s[i] + j)
    return str_list


str_list = str_sort("123")
print(str_list)

组合

import itertools

# itertools.combinations()
def combinations(iterable, r):
    pool = tuple(iterable)
    n = len(pool)
    if r > n:
        return

    indices = list(range(r))
    yield tuple(pool[i] for i in indices)
    while True:
        for i in reversed(range(r)):
            if indices[i] != i + n - r:
                break
        else:
            return

        indices[i] += 1
        for j in range(i + 1, r):
            indices[j] = indices[j - 1] + 1
        yield tuple(pool[i] for i in indices)


if __name__ == '__main__':
    # combinations('ABCD', 2) --> AB AC AD BC BD CD
    # combinations(range(4), 3) --> 012 013 023 123
    for i in combinations(range(1, 5), 2):
        print(i)