Permutations [LeetCode]

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

Notes: recursive, in place.

 1 class Solution {
 2 public:
 3     vector<vector<int> > permute(vector<int> &num) {   
 7         int size = num.size();
 8         vector<vector<int> > result;
 9         if(size == 0)
10             return result;
11         if(size == 1) {
12             vector<int> per;
13             per.push_back(num[0]);
14             result.push_back(per);
15             return result;
16         }
17         
18         for(int i = 0; i < size; i ++) {
19             int current = num[i];
20             num.erase(num.begin() + i);
21             vector<vector<int> > ret = permute(num);
22             num.insert(num.begin() + i, current);
23             for(auto item: ret) {
24                 item.insert(item.begin(), current);
25                 result.push_back(item);
26             }
27         }
28         return result;
29     }
30 };