面试题 08.04：幂集

输入： nums = [1,2,3]
 输出：
[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

class Solution {
public:
    vector<vector<int>> res;
    vector<int> path;
    void backtracing(vector<int>& nums,int startIndex){
        if(startIndex>nums.size()) return;
        else res.push_back(path);
        for(int i=startIndex;i<nums.size();i++){
            path.push_back(nums[i]);
            backtracing(nums,i+1);
            path.pop_back();
        }
    }
    vector<vector<int>> subsets(vector<int>& nums) {
        backtracing(nums,0);
        return res;
    }
};