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

if(l1== NULL) return l2;

if(l2 == NULL) return l1;

int farward = 0;

ListNode *head = new ListNode();

ListNode* tail = head;

while(l1 || l2) {

int n1 = l1? l1-> val:0;

int n2 = l2? l2-> val:0;

int i = n1 + n2 + farward;

farward = i >= 10? 1:0;

tail -> next =new ListNode(i%10);

if(l1 != NULL)l1 = l1 -> next;

else l1 = NULL;

if(l2 != NULL)l2 = l2 -> next;

else l2 = NULL;

tail = tail -> next;

}

if(farward == 1) {

tail-> next = new ListNode(1);

tail -> next -> next = NULL;

}

return head-> next;

}