组合总和3

解题思路：递归回溯

class Solution {    public List
     
      
       > result = new ArrayList
       
        
         >();    public List
         
          
           > combinationSum3(int k, int n) { List
           
             list = new ArrayList<>(); combinationSum3(1,k,n,list); return result; } public void combinationSum3(int start, int k, int n, List
            
              list) { if(k==0) { if(n==0) { List
             
               newList = (List)((ArrayList)list).clone(); result.add(newList); } return; } for(int i=start;i<10;++i) { list.add(i); combinationSum3(i+1,k-1,n-i,list); list.remove((Integer)i); } }}