/**
 * 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) {
        ListNode *L = l1,*pre,*old;
        int carry_bit = 0
 
,t;
         while (l1 && l2){
             pre = l1, old = l2;
            t = l1->val + l2->val + carry_bit;
            carry_bit = t/10;
            l1->val = t % 10;
            l1 = l1->next, l2 = l2->next;
            delete(old);
         }
         if (l2) l1=pre->next = l2;
         while
 
 (l1) {
             pre = l1;
             t = l1->val + carry_bit;
             carry_bit = t / 10;
             l1->val = t % 10;
             l1 = l1->next;
         }
         if (carry_bit) {
             old = new ListNode(1);
             pre->next = old;
         }
         return L;
    }
};