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 sub(vector<vector<int> > & ret,vector<int>& num,int index){
        if(index==num.size()){
            ret.push_back(num);
            return ;
        }
        int temp;
        for(int i=index;i<num.size();i++){
            temp=num[i];
            num[i]=num[index];
            num[index]=temp;
            sub(ret,num,index+1);
            temp=num[i];
            num[i]=num[index];
            num[index]=temp;
        }
    }
    vector<vector<int> > permute(vector<int> &num) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<vector<int> >ret;
        sub(ret,num,0);
        return ret;
    }
};