2.cpp

/* You are given two non - empty linked lists representing two non - negative integers.
* The digits are stored in reverse order, and each of their nodes contains a single digit.
* Add the two numbersand return the sum as a linked list.
*/

/*
 * 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;
        ListNode head;
        ListNode* sum = &head;

        while (l1 || l2) {
            int val = carry + (l1 ? l1->val : 0) + (l2 ? l2->val : 0);
            carry = val / 10;
            sum->next = new ListNode(val % 10);
            sum = sum->next;
            l1 = (l1 ? l1->next : nullptr);
            l2 = (l2 ? l2->next : nullptr);
        }
        if (carry) {
            sum->next = new ListNode(carry);
        }

        return head.next;
    }
};