LeetCode: Permutations

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].

class Solution {
public:
    void perm(vector<int> &num, vector<vector<int> >&result,
              int nStart, int nEnd)
    {
        if (nStart < nEnd-1)
        {
            perm(num, result, nStart+1, nEnd);
            for (int i = nStart + 1; i < nEnd; ++i)
            {
                int tmp = num[nStart];
                num[nStart] = num[i];
                num[i] = tmp;
                
                perm(num, result, nStart+1, nEnd);
                
                num[i] = num[nStart];
                num[nStart] = tmp; 
            }
        }
        else
        {
            result.push_back(num);
        }
    }
    
    vector<vector<int> > permute(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> >result;
        perm(num, result, 0, num.size());
        return result;
    }
};