2-Add-Two-Numbers.js

/**
 * 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}
 */
const addTwoNumbers = (l1, l2) => {
  const head = new ListNode(0);
  let carry = 0,
    sum = 0,
    now = head,
    a = l1,
    b = l2;

  while (a !== null || b !== null) {
    sum = (a ? a.val : 0) + (b ? b.val : 0) + carry;
    carry = Math.floor(sum / 10);
    now.next = new ListNode(sum % 10);
    now = now.next;
    a = a ? a.next : null;
    b = b ? b.next : null;
  }

  if (carry) now.next = new ListNode(carry);

  return head.next;
};