/**
1. 和归并排序类似, l1链存储结果, 如果l1 较短就把l2后面接上即可
2. 进位可能多一位, 要在最后new出来
*/


/**
 * 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 ? l2 : l1;

        int next = 0;
        ListNode prev = l1, head  = l1;
        while (l1 != null && l2 != null){
            int sum = l1.val + l2.val + next;
            l1.val = sum % 10;
            next = sum / 10;

            prev = l1;
            l1 = l1.next;
            l2 = l2.next;
        }

        if (l2 != null) {
            prev.next = l2;
            l1 = l2;
        }

        while (l1 != null && next != 0) {
            int sum = l1.val + next;
            l1.val = sum % 10;
            next = sum / 10;
            prev = l1;
            l1 = l1.next;
        }

        if (next != 0) {
            prev.next = new ListNode(next);
        }
        return head;
    }

}