vector<vector<int>> subsets(vector<int>& nums) {
auto ret = vector<vector<int>>();
auto tmp = vector<int>();
backtrack(ret, tmp, nums, 0);
void backtrack(vector<vector<int>>& list, vector<int>& tempList, vector<int>& nums, int start) {
list.push_back(tempList);
for (auto i = start; i < nums.size(); ++i) {
tempList.push_back(nums[i]);
backtrack(list, tempList, nums, i + 1);