/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function(l1, l2) {
  
  var carry = 0;
  var sum;
  var startNode = null;
  var prevNode = null;
  var node = null;
  
  while(true){
    
    if (l1 !== null){
      if (l2 !== null){
        sum = l1.val + l2.val + carry;
      } else {
        sum = l1.val + carry;
      }
    } else {
      if (l2 === null){
        break;
      } else {
        sum = l2.val + carry;
      }
    }
    
    if (sum >= 10){
      sum = sum - 10;
      carry = 1;
    } else {
      carry = 0;
    }
    
    node = {
      val: sum,
      next: null
    }
    
    if (prevNode !== null){
      prevNode.next = node;
    }
    
    if (startNode === null){
      startNode = node;
    }
    
    prevNode = node;
    if (l1 !== null){
      l1 = l1.next;
    }
    if (l2 !== null){
      l2 = l2.next;
    }
    
  }
  
  if (carry === 1){
    node = {
      val: 1,
      next: null
    }
    
    prevNode.next = node;
  }
  
  return startNode;
};