Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,

If n = 4 and k = 2, a solution is:

[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

public class Solution {    List
     
      
       > res;    List
       
         seq;    public List
        
         
          > combine(int n, int k) {        //注意分析题意，可选数组时从1到n，因此起始findCom函数的start参数需要从1开始        res=new ArrayList
          
           
            >(); seq=new ArrayList
            
             (); findCom(n,1,0,k); return res; } public void findCom(int n,int start,int level,int k){ if(level==k){ res.add(new ArrayList
             
              (seq)); return; } for(int i=start;i<=n;i++){ //注意i可以为n seq.add(i); findCom(n,i+1,level+1,k); seq.remove(seq.size()-1); } }}