描述
You are given two linked lists representing two non-negative numbers. e 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
 

class Solution{
public:
    ListNode *addTwoNumbers(ListNode *l1,ListNode *l2){
        ListNode head(-1);
        int carry = 0;
        ListNode *prev = &head;
        for(ListNode *pa = l1, *pb = l2;
            pa != nullptr || pb != nullptr;
            pa = pa == nullptr ? nullptr : pa->next,
            pb = pb == nullptr ? nullptr : pb->next,
            prev = prev->next){
            const int ai = pa == nullptr ? 0 : pa-val;
            const int bi = pb == nullptr ? 0 : pb->val;
            const int value = (ai + bi + carry) % 10;
            carry = (ai + bi + carry) / 10;
            prev->next = new ListNode(value);
        }
        if(carry > 0)
            prev->next = new ListNode(carry);
        return head.next;
    }
};

参考资料：

LeetCode题解