add-two-numbers.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2, int carry = 0) {
        if (l1 == NULL && l2 == NULL && carry == 0) {
            return NULL;
        }
        int sum = (l1 ? l1->val : 0) + (l2 ? l2->val : 0) + carry;
        ListNode* res = new ListNode(sum % 10);
        res->next = addTwoNumbers((l1 ? l1->next : NULL),
                                  (l2 ? l2->next : NULL), sum / 10);
        return res;
    }
};