/**
 * 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 == NULL) return l2;
        if(l2 == NULL) return l1;
        
        int flag = 0;     //進位標誌
        ListNode* tail = new ListNode(0); 
        ListNode* ptr = tail; //?????


        while(l1 != NULL || l2 != NULL){
            int val1 = 0;
            if(l1 != NULL){
                val1 = l1->val;
                l1 = l1->next;
            }


            int val2 = 0;
            if(l2 != NULL){
                val2 = l2->val;
                l2 = l2->next;
            }


            int tmp = val1 + val2 + flag;
            ptr->next = new ListNode(tmp % 10);
            flag = tmp / 10;
            ptr = ptr->next;
        }


        if(flag == 1){
            ptr->next = new ListNode(1);
        }
        return tail->next; //??????????????/
    }
};