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.
Examples :

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

This problem was taken from Leetcode

The solution:

Java Script

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */



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) {
  
  function doSum(a,b, carry) {
    var result = {
      val: 0,
      carry: 0
    }
    var sum = a + b + carry;
    if(sum > 9) {
      result.val = sum - 10;
      result.carry = 1;
    }
    else {
      result.val = sum;
      result.carry = 0;
    }
    return result;
  }

  var nodeA = l1;
  var nodeB = l2;
  var end = false;
  var carry = 0;
  var result = new ListNode(0, null);
  var resultPointer = result;
  var tmp = null;

  while(nodeA != null || nodeB != null) {
    var valA = nodeA == null ? 0 : nodeA.val;
    var valB = nodeB == null ? 0 : nodeB.val;
    var sumResult = doSum(valA, valB, carry);
    carry = sumResult.carry;
    
    resultPointer.val = sumResult.val;
    resultPointer.next = new ListNode(0, null);
    tmp = resultPointer;
    resultPointer = resultPointer.next;

    nodeA = nodeA ? nodeA.next : null;
    nodeB = nodeB ? nodeB.next : null;
  }

  if(carry != 0) {
    tmp.next.val = carry; // add carry to the val of the last node
  }
  else 
    tmp.next = null; // remove the last empty node.
  return result;
};




var l1 = new ListNode(9, 
          new ListNode(9,
          new ListNode(9,
          new ListNode(9, null)
      ))
);

var l2 = new ListNode(9, 
          new ListNode(9,
          new ListNode(9,
          new ListNode(9,
          new ListNode(9,
          new ListNode(9,                    
          new ListNode(9, null)
      )))))
)

// [9,9,9] , [9,9,9,9,9]
// result: [8,9,9,0,0,1]

var result = addTwoNumbers(l1, l2);

console.log(result);