addTwoNumbers.cpp

#include <iostream>
using namespace std;

// 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;
        ListNode *tail = &dummy;
        int carry = 0;

        while (l1 || l2 || carry)
        {
            int x = l1 ? l1->val : 0;
            int y = l2 ? l2->val : 0;
            int sum = carry + x + y;
            int digit = sum % 10;
            carry = sum / 10;
            tail->next = new ListNode(digit);
            tail = tail->next;
            if (l1)
            {
                l1 = l1->next;
            }
            if (l2)
            {
                l2 = l2->next;
            }
        }
        return dummy.next;
    }
};