2 两数相加

    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        if (l1->next == nullptr && l1->val == 0)
            return l2;
        else if (l2->next == nullptr && l2->val == 0)
            return l1;
        int carry = 0;
        ListNode head(0), *temp = &head;
        while (l1 != nullptr && l2 != nullptr) {
            l1->val = l1->val + l2->val + carry;
            if (l1->val > 9) {
                carry = 1;
                l1->val %= 10;
            } else
                carry = 0;
            temp->next = l1;
            temp = temp->next;
            l1 = l1->next;
            l2 = l2->next;
        }
        if (carry) {
            ListNode *it = (l1 != nullptr) ? l1 : l2 != nullptr ? l2 : nullptr;
            if (it != nullptr) {
                while (true) {
                    it->val += carry;
                    temp->next = it;
                    temp=temp->next;
                    if (it->val == 10) {
                        carry = 1;
                        it->val = 0;
                        if (it->next != nullptr)
                            it = it->next;
                        else {
                            temp->next = new ListNode(1);
                            break;
                        }
                    } else
                        break;
                }
            } else
                temp->next = new ListNode(1);
        } else
            temp->next = (l1 != nullptr) ? l1 : l2 != nullptr ? l2 : nullptr;
        return head.next;
    }