这个题目目的是为了求所有的子集，方法是遍历每个数字加进集合，比如最开始是[]，加入1变成[1]，再加上原来有的[]，变成[] [1]，遍历到2 ，加上2 变成[2] [1,2]加上原来的[] [1] 变成[][1] [2] [1,2]，代码如下：

class Solution(object):
    def subsets(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        res = []
        if len(nums) == 0:
            return res
        res.append([])
        for n in nums:
            temp = []
            for t in res:
                m = t + [n]
                temp.append(m)
            res = res + temp
        return res

回溯法：

class Solution(object):
    def subsets(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        if len(nums) == 0:
            return []
        res = []
        temp = []
        self.hs(nums, res, temp, 0)
        return res
    def hs(self, nums, res, temp, i):
        if i == len(nums):
            res.append(temp)
            return
        self.hs(nums, res, temp+[nums[i]], i+1)
        self.hs(nums, res, temp, i+1)