leetcode[46]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, int i, int N, vector<vector<int>> &res)
{
    if(i==N)
        res.push_back(num);
    for (int j=i;j<N;j++)
    {
        swap(num[i],num[j]);
        perm(num,i+1,N,res);
        swap(num[j],num[i]);
    }
}
vector<vector<int> > permute(vector<int> &num) 
{
    vector<vector<int> > res;
    perm(num,0,num.size(),res);
    return res;
}
};