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;
        
    }
};