2. Add Two Numbers (Medium)

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 contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 -> 8

Solution: Linked List + Math

ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
    ListNode* head = new ListNode(0); // dummy head
    ListNode* cur = head;
    int carrier = 0;
    while (l1 || l2 || carrier) {
        int val = (l1 ? l1->val:0) + (l2 ? l2->val:0) + carrier;
        carrier = val / 10;
        cur->next = new ListNode(val%10);
        l1 = l1 ? l1->next:l1;
        l2 = l2 ? l2->next:l2;
        cur = cur->next;
    }

    return head->next;
}