LeetCode 2. 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]

Constraints:

The number of nodes in each linked list is in the range [1, 100].
0 <= Node.val <= 9
It is guaranteed that the list represents a number that does not have leading zeros.

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function(l1, l2) {
    let head = new ListNode();
    let temp = head;
    let sum = 0;
    let carry = 0;
    while (l1 !== null || l2 !== null) {
        sum = carry + (l1 !== null ? l1.val : 0) + (l2 !== null ? l2.val : 0);
        carry = sum >= 10 ? 1 : 0;
        sum = sum % 10;
        
        temp.next = new ListNode(sum);
        temp = temp.next;
        
        if (l1) l1 = l1.next;
        if (l2) l2 = l2.next;
        
        if (carry) temp.next = new ListNode(carry);
    }
    return head.next;
};