Question：

Given a set of distinct integers, nums, return all possible subsets.

Note: The solution set must not contain duplicate subsets.

For example,

If nums = [1,2,3], a solution
is:

[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

Algorithm：

也是用二叉树的思想，不过这次不需要判断长度，每更新一次数组，就赋值给二维数组（最终结果）。如果数组长度等于nums的长度就返回。

Accepted Code：

class Solution {
public:
    vector<vector<int>> res;
    vector<vector<int>> subsets(vector<int>& nums) {
        vector<int> cur;
        BFS(nums,cur,0);

        return res;
    }
    void BFS(vector<int>& nums,vector<int>& cur,int k)
    {
        if(cur.size()==nums.size())
        {
            res.push_back(cur);
            return;
        }
        else
        {
            res.push_back(cur);
            for(int i=k;i<nums.size();i++)
            {
                vector<int> temp=cur;
                temp.push_back(nums[i]);
                BFS(nums,temp,i+1);
            }
        }
    }
};