我的递归有什么问题？ 尝试在每次遍历中添加两个节点

Question

Seems like my linear linked list didn't connect for each recursion. Need explanation why my code doesn't work.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */


public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null)
            return null;
        int val = l1.val + l2.val;
        ListNode newNode = new ListNode(val);
        System.out.println(newNode.val);
        return newNode.next = addTwoNumbers(l1.next, l2.next);
    }
}

Your input

[2,4,3] [5,6,4]

Your stdout

7 10 7

Your answer

[]

Expected answer

[7,10,7]

Answer 1

I think that you didn't return newNode.next . You must call recursion method with newNode.next but you must return newNode . Ps. Remember to add if (l2==null) return null;

Answer 2

Change

return newNode.next = addTwoNumbers(l1.next, l2.next);

To

newNode.next = addTwoNumbers(l1.next, l2.next);
return newNode