https://leetcode.com/problems/add-two-numbers/

/**
 * 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 dummy(0);

        ListNode* curr = &dummy;

        int carry =0 ;

        while(l1 || l2 || carry){
            int val1 = (l1 ? l1->val : 0);
            int val2 = (l2 ? l2->val : 0);

            int sum = val1 + val2 + carry;

            carry = sum/10;
            curr->next = new ListNode(sum%10);
            curr = curr->next;

            if(l1) l1 = l1->next;
            if(l2) l2 = l2->next;
        }
        return dummy.next;
    }
};

In summary: