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.

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {   
        int size = num.size();
        vector<vector<int> > result;
        if(size == 0)
            return result;
        if(size == 1) {
            vector<int> per;
            per.push_back(num[0]);
            result.push_back(per);
            return result;
        }
        
        for(int i = 0; i < size; i ++) {
            int current = num[i];
            num.erase(num.begin() + i);
            vector<vector<int> > ret = permute(num);
            num.insert(num.begin() + i, current);
            for(auto item: ret) {
                item.insert(item.begin(), current);
                result.push_back(item);
            }
        }
        return result;
    }
};