You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null || l2 == null){
            return l1 != null ? l1:l2;
        }
        
        int sum = l1.val + l2.val;
        ListNode result = new ListNode(sum % 10);
        result.next = addTwoNumbers(l1.next,l2.next);
            
        if(sum >= 10){//需要進位的情況
            result.next = addTwoNumbers(result.next,new ListNode(sum / 10));
        }

        return result;
    }
}
 

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