/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int sum = 0;
        ListNode head = l1;
        while (l1 != null) {            
            if (l2 != null) {
                sum = l1.val + l2.val;
                l2 = l2.next;
                if(l1.next == null ){
                    l1.next = l2;
                    l2 = null;
                }
            }else{
                sum = l1.val;
                if(sum / 10 ==0){
                    break;
                }
            }
            l1.val = sum % 10;
            if( sum / 10>0){
                if(l1.next==null){
                    l1.next = new ListNode(sum/10);
                    break;
                }else{
                    l1.next.val = l1.next.val + sum / 10;
                }
            }
            l1 = l1.next;
        }
        return head;
    }
}

参考文章