2. Add Two Numbers

Problem #2: https://leetcode.com/problems/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 contains a single digit. Add the two numbers and return the sum as a linked list.

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

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]
Output: [0]

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

Solution

/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function(l1, l2) {
    let node = null;
    const carry = arguments[2];

    if (l1 || l2) {
        if (!l1) {
            l1 = new ListNode();
        }
        if (!l2) {
            l2 = new ListNode();
        }
        let val = l1.val + l2.val;
        if (carry) {
            val += 1;
        }
        node = new ListNode(val %10, null);
        node.next = addTwoNumbers(l1.next, l2.next, val > 9 ? 1 : 0);
    } else if (carry) {
        node = new ListNode(1, null);
    }
    return node;
};