class Solution {
public:
    vector<vector<int>> res;
    vector<vector<int>> combine(int n, int k) {
        vector<int> temp;
        vector<int> nums;
        // 构建一个nums数组避免下标0数字1这些判断，避免混乱
        for(int i = 1; i <= n; i++){
            nums.push_back(i);
        } 
        back(nums,k,0,temp);
        return res;

    }

    void back(vector<int> &nums, int k, int index, vector<int> &temp){
     
        if(temp.size()==k){
            res.push_back(temp);
            // return;
        }
        
        for(int i = index; i < nums.size(); i++){
            temp.push_back(nums[i]);
            back(nums,k,i+1,temp);
            temp.pop_back();  // 回溯
        }
    }
};