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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

/**
 * 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) {
    if (!l1 && !l2) { return;}
    var l3 = new ListNode(0);
    var p = l3;
    var extra = 0;
    while(l1 || l2 || extra) {
        var sum = (l1?l1.val:0) + (l2?l2.val:0) +extra;
        extra = parseInt(sum/10);
        p.next = new ListNode(sum%10);
        p = p.next;
        l1 = l1?l1.next:l1;
        l2 
 
= l2?l2.next:l2;
    }
    return l3.next;
};

leetcode2. Add Two Numbers