339. Nested List Weight Sum 递归的base case是什么

大家好，这道题不好debug,因为好多写好的api
有一件事不明，就是如果输入是[[1,1],2,[ ]]的时候，这个递归的程序怎样处理空list的？返回的是null吗？
进而，这个recursion的base case是什么？ 貌似不像是树那样if(root==null) return 逻辑：

1. /**
   
2. * // This is the interface that allows for creating nested lists.
   
3. * // You should not implement it, or speculate about its implementation
   
4. 
   
5. * public interface NestedInteger {
   
6. *     // Constructor initializes an empty nested list.
   
7. *     public NestedInteger();
   
8. *
   
9. *     // Constructor initializes a single integer.
   
10. *     public NestedInteger(int value);
    
11. *
    
12. *     // [url=home.php?mod=space&uid=160137]@return[/url] true if this NestedInteger holds a single integer, rather than a nested list.
    
13. *     public boolean isInteger();
    
14. *
    
15. *     // @return the single integer that this NestedInteger holds, if it holds a single integer
    
16. *     // Return null if this NestedInteger holds a nested list
    
17. *     public Integer getInteger();
    
18. *
    
19. *     // Set this NestedInteger to hold a single integer.
    
20. *     public void setInteger(int value);
    
21. *
    
22. *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
    
23. *     public void add(NestedInteger ni);
    
24. *
    
25. *     // @return the nested list that this NestedInteger holds, if it holds a nested list
    
26. *     // Return null if this NestedInteger holds a single integer
    
27. *     public List<NestedInteger> getList();
    
28. * }
    
29. */
    
30. class Solution {
    
31.     public int depthSum(List<NestedInteger> nestedList) {
    
32.         if(nestedList == null) return 0;
    
33.         return helper(nestedList,1);
    
34.     }
    
35.     public int helper(List<NestedInteger> nestedList, int level){
    
36.         int sum=0;
    
37.         for(NestedInteger nest: nestedList){
    
38.             if(nest.isInteger()){
    
39.                 sum+=nest.getInteger() * level;
    
40.             }else{
    
41.                 sum+= helper(nest.getList(),level+1);//所以这个recursion没有base case吗
    
42.             }
    
43.         }
    
44.         return sum;
    
45.     }
    
46. }

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求教，谢谢

base case就是nestedList为空的情况，此时helper内的for循环不执行，直接返回sum = 0