/**

  * Definition for singly-linked list.

  * struct ListNode {

  *     int val;

  *     ListNode *next;

  *     ListNode(int x) : val(x), next(NULL) {}

  * };

  */

 class Solution {

 public:

     ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {

         if (!l1 || !l2) {

             return NULL;

         }

         int carry = ;  //进位

         ListNode * result = new ListNode();

         ListNode * first = result;  //追踪结果链表的当前节点

         ListNode * pre = NULL;  //追踪结果链表的前一个节点

         //当l1和l2均没有超过链表末尾节点时

         while (l1 && l2) {

             first->val = (carry + l1->val + l2->val) % ;

             carry = (carry + l1->val + l2->val) / ;

             if (pre == NULL)

                 pre = first;

             else {

                 pre->next = first;

                 pre = first;

             }

             first = new ListNode();

             l1 = l1->next;

             l2 = l2->next;

         }

         //当l1和l2都超过链表末尾节点时

         if (!l1 && !l2) {

             if (carry == ) {

                 first->val = carry;

                 pre->next = first;

             }

             return result;

         }

         //当l1超过末尾而l2尚未超过时

         if (!l1 && l2) {

             while (l2) {

                 first->val = (carry + l2->val) % ;

                 carry = (carry + l2->val) / ;

                 if (pre == NULL)

                     pre = first;

                 else {

                     pre->next = first;

                     pre = first;

                 }

                 first = new ListNode();

                 l2 = l2->next;

             }

             if (carry == ) {

                 first->val = ;

                 pre->next = first;

             }

             return result;

         }

         //当l2超过末尾而l1尚未超过时

         if (!l2 && l1) {

             while (l1) {

                 first->val = (carry + l1->val) % ;

                 carry = (carry + l1->val) / ;

                 if (pre == NULL)

                     pre = first;

                 else {

                     pre->next = first;

                     pre = first;

                 }

                 first = new ListNode();

                 l1 = l1->next;

             }

             if (carry == ) {

                 first->val = ;

                 pre->next = first;

             }

             return result;

         }

     }

 };