addTwoNumbers.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) {
        ListNode* dummyHead = new ListNode(0);
        ListNode* curr = dummyHead;
        int carry = 0;
        while (l1 != NULL || l2 != NULL || carry != 0) {
            int x = l1 ? l1->val : 0;
            int y = l2 ? l2->val : 0;
            int sum = carry + x + y;
            carry = sum / 10;
            curr->next = new ListNode(sum % 10);
            curr = curr->next;
            l1 = l1 ? l1->next : nullptr;
            l2 = l2 ? l2->next : nullptr;
        }
        return dummyHead->next;
    }
};