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 };