给定一个整数n，和k，返回包含k个所有所有子数组

import java.util.*;public class Solution {    ArrayList<ArrayList<Integer>> result = new ArrayList<>();    public ArrayList<ArrayList<Integer>> combine(int n, int k) {       List<Integer> list=new ArrayList<Integer>();         backtracking(n,k,1,list);         return result;     }    public void backtracking(int n,int k,int start,List<Integer>list){         if(k<0) return ;         else if(k==0){             result.add(new ArrayList(list));         }else{             for(int i=start;i<=n;i++){                  list.add(i);                  backtracking(n,k-1,i+1,list);                  list.remove(list.size()-1);              }         }      }  }