偷懒写法（next_permutation nb！）

class Solution {
public:
    vector<vector<int>> ans;
    vector<vector<int>> permutation(vector<int>& nums) {
        do
        {
            ans.push_back(nums);
        }while(next_permutation(nums.begin(), nums.end()));                             // 全排列 
        return ans;
    }
};