Leetcode: Add two numbers

leetcode-add-two-numbers.js

// https://leetcode.com/submissions/detail/508355858/
/**
 * 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 result;
  let prev;

  let carry = 0;
  while (l1 || l2) {
    const a = l1 ? l1.val : 0;
    const b = l2 ? l2.val : 0;
    l1 = l1 ? l1.next : null;
    l2 = l2 ? l2.next : null;
    let s = a + b + carry;

    if (s > 9) {
      s = s - 10;
      carry = 1;
    } else {
      carry = 0;
    }

    if (!prev) {
      prev = new ListNode(s);
      result = prev;
    } else {
      const node = new ListNode(s);
      prev.next = node;
      prev = node;
    }
  }

  if (carry !== 0) {
    const node = new ListNode(carry);
    prev.next = node;
  }

  return result;
};