子集

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xv67o6/

cost: 6m33s

class Solution {
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        vector<vector<int>> res;
        vector<int> vec;
        this->walk(res, nums, 0, vec);
        return res;
    }

private:
    void walk(vector<vector<int>>& res, vector<int>& nums, int k,
              vector<int>& vec) {
        if (nums.size() == k) {
            res.push_back(vec);
            return;
        }
        this->walk(res, nums, k + 1, vec);
        vec.push_back(nums[k]);
        this->walk(res, nums, k + 1, vec);
        vec.pop_back();
    }
};