Add Two Numbers

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.

Example Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8 Explanation: 342 + 465 = 807.

Solution:

We create a dummy node to save the new linked list. And we can do this in one single while loop.

Note: The key is the while loop condition

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

    return dummyHead->next;
}