【second】Add Two Numbers

    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int carry = 0;
        ListNode* nhead=NULL,*prev=NULL;
        ListNode* it1=l1,*it2=l2;
        while(it1||it2||carry)
        {
            int num1 = (it1?it1->val:0);
            int num2 = (it2?it2->val:0);
            int sum = num1+num2+carry;
            if(nhead==NULL)
                prev = nhead = new ListNode(sum%10);
            else
                {
                    prev->next = new ListNode(sum%10);
                    prev = prev->next;
                }
            carry = sum/10;
            it1 = (it1?it1->next:NULL);
            it2 = (it2?it2->next:NULL);
        }
        
        return nhead;
    }