2. Add Two Numbers

/**
 * 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) {
        int sign = 0, sum = 0;
        ListNode result = new ListNode(0);  // 头结点
        ListNode head = result;
        while(l1 != null && l2 != null) {
            sum = l1.val + l2.val + sign;
            sign = sum / 10;
            result.next = new ListNode(sum%10);
            result = result.next;
            l1 = l1.next;
            l2 = l2.next;
        }

        while(l1 != null) {
            sum = l1.val + sign;
            sign = sum / 10;
            result.next = new ListNode(sum%10);
            result = result.next;
            l1 = l1.next;
        }
        while(l2 != null) {
            sum = l2.val + sign;
            sign = sum / 10;
            result.next = new ListNode(sum%10);
            result = result.next;
            l2 = l2.next;
        }
        if(sign > 0)　　// 最后可能有进位
            result.next = new ListNode(sign);
        return head.next;
    }
}