LeetCode Problem : Permutations

Problem

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].

Code

void permute_r(vector<int> num,vector<int> &result,int l,int n,int pos,
        vector<vector<int> > &res){
    if(pos == n){
        res.push_back(vector<int>(result.begin(),result.begin() + pos));
        return;
    }
    for(int i = l;i < n;++i){
        swap(num[l],num[i]);
        result[pos] = num[l];
        permute_r(num,result,l + 1,n,pos + 1,res);
    }
}
vector<vector<int> > permute(vector<int> &num) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    vector<vector<int> > res;
    int n = num.size();
    vector<int> result(n);
    permute_r(num,result,0,n,0,res);
    return res;
}