class Solution:
    def permute(self, nums: List[int]) -> List[List[int]]:
        def gen(i: int, n: int, mask: int) -> List[List[int]]:
            if i == n:
                return [[None for _ in range(n)]]
            res = []
            for bit in range(n):
                if (mask >> bit) & 1:
                    continue
                else:
                    cur = gen(i + 1, n, mask | (1 << bit))
                    for v in cur:
                        v[bit] = i
                        res.append(v.copy())
            return res

        n = len(nums)
        perms = gen(0, n, 0)
        for i in range(len(perms)):
            for j in range(len(perms[i])):
                perms[i][j] = nums[perms[i][j]]
        return perms

https://leetcode.com/problems/permutations/