leetcode Ch3-DFS & Backtracking II

 1 class Solution
 2 {
 3 public:
 4     vector<int> grayCode(int n)
 5     {
 6         vector<int> result={0};
 7         if(n==0) return result;
 8         return dfs(n);
 9     }
10     vector<int> dfs(int n)
11     {
12         if(n==1)
13         {
14             vector<int> v={0,1};
15             return v;    
16         }
17         vector<int> tmp=dfs(n-1);
18         int len=tmp.size();
19         for(int i=len-1;i>=0;i--)
20         {
21             tmp.push_back(tmp[i]+len);    
22         }
23         return tmp;
24     }
25 };