20210717

子集
题目链接：https://leetcode-cn.com/problems/subsets/

思路：求幂集的思想就是对数组中的每一个元素都做出包含/不包含的选择，从第一个一直选到最后一个，每确定一个子集后就回溯到上一层。

参考代码：

class Solution {
private:
    vector<int> subRes;
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        vector<vector<int>> res;
        if (nums.size() == 0) {
            return res;
        }
        recursiveSubsets(nums, res, 0);
        return res;
    }
    void recursiveSubsets(vector<int>& nums, vector<vector<int>>& res, int i) {
        if (i == nums.size() - 1) {
            res.push_back(subRes);
            subRes.push_back(nums[i]);
            res.push_back(subRes);
            subRes.pop_back();
            return;
        }
        recursiveSubsets(nums, res, i + 1);
        subRes.push_back(nums[i]);
        recursiveSubsets(nums, res, i + 1);
        subRes.pop_back();
    } 
};