Add Two Numbers

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

代码：

class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;
        
        ListNode * dummy = new ListNode(-1);
        int carry = 0;
        ListNode *p = dummy;
        
        for(; l1 || l2; p = p->next){
            int opr1 = l1?l1->val:0;
            int opr2 = l2?l2->val:0;
            
            int sum = (opr1 + opr2 + carry)%10;
            carry = (opr1 + opr2 + carry)/10;
            
            //take advantage of existing list node
            p->next = l1?l1:l2;
            p->next->val = sum;
            
            l1 = l1?l1->next:NULL;
            l2 = l2?l2->next:NULL;
        }
        
        if(carry != 0){
            p->next = new ListNode(carry);
        }
        
        return dummy->next;
    }
};