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 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.

leetcode-2-add-two-integers-from-linked-lists.js

const addTwoNumbers = (l1, l2) => {
  if(l1 === null && l2 === null) {
      return null;
  }
  if(l1 === null) return l2;
  if(l2 === null) return l1;

  let n1, n2, n, carry=0;
  let l = new ListNode(); //our result 
  let pl = l; //pointer to run through l
  
  while(l1 || l2){
    n1 = l1 === null ? 0 : l1.val;
    n2 = l2 === null ? 0 : l2.val;
    n = n1 + n2 + carry;
    carry = n>=10 ? 1 : 0;
    n = n%10;
    pl.val = n;
    //we need to make sure that it's only added if it's not yet the end
    if((l1 && l1.next) || (l2 && l2.next)) {
        pl.next = new ListNode();
        pl = pl.next;
    }
    if(l1 !== null) l1 = l1.next;
    if(l2 !== null) l2 = l2.next;
  }
  
  if(carry > 0)
    pl.next = new ListNode(carry);

  return l;
};