leetcode:Permutations

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