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> result = new ArrayList<>();
    public ArrayList> combine(int n, int k) {
       List list=new ArrayList();  
       backtracking(n,k,1,list);  
       return result; 
    }
    public void backtracking(int n,int k,int start,Listlist){  
       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);  
            }  
       }  
    }  
}