题目描述：

方法一：回溯

class Solution:    def subsets(self, nums: List[int]) -> List[List[int]]:        if not sum:            return []        res = []        n = len(nums)        def backtrack(idx,temp_list):            res.append(temp_list)            for i in range(idx,n):                backtrack(i+1,temp_list+[nums[i]])        backtrack(0,[])        return res

方法二：迭代

class Solution:     def subsets(self, nums: List[int]) -> List[List[int]]:         res = [[]]         for i in nums:             res = res + [[i] + num for num in res]         return res