全排列

class Solution:
    def permute(self, nums: list[int]) -> list[list[int]]:
        res = []
        track = []
        n = len(nums)
        used = [0] * n
        def backtrack():
            if len(track)==n:
                s = copy.deepcopy(track)
                res.append(s)
            for i in range(n):
                if not used[i]:
                    used[i] = 1
                    track.append(nums[i])
                    backtrack()
                    track.remove(nums[i])
                    used[i] = 0
        backtrack()
        return  res