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;

}
};