Permutations 全排列 回溯

class Solution {
private:
    vector<vector<int> > ret;
public:
    void perm(vector<int> num,int i){
        if(i==num.size()){
            ret.push_back(num);
            return;
        }
        for(int j=i;j<num.size();j++){
            swap(num[i],num[j]);            
            perm(num,i+1);                  
            swap(num[j],num[i]);           //复原，进行下一个交换前需复原之前状态
        }
    }
    vector<vector<int> > permute(vector<int> &num) {
        perm(num,0);
        return ret;
    }
};