子集

class Solution {
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        vector<vector<int>> ans;
        for (int num : nums) {
            int n = ans.size();
            for (int i = 0; i < n; ++i) {
                ans.push_back(ans[i]);
                ans[i].push_back(num);
            }
            ans.push_back({ num });
        }
        ans.push_back({});
        return ans;
    }
};