/*

* Difficulty:

* Medium

*

* Desc:

* 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.

*

*

* Example:

* Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

* Output: 7 -> 0 -> 8

*

* 简而言之，就是将数字作为链表储存起来，并且是倒序储存。比如 123 储存为 3 -> 2 -> 1

* 现在两个数字相加，求其和的链表（用数组表示）

*/

/**

* 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 l = new ListNode(-999);

var nextList = l;

var upper = 0;

while(l1 || l2) {

var value1 = l1 ? l1.val : 0;

var value2 = l2 ? l2.val : 0;

var sum = value1 + value2 + upper;

var val = sum % 10;

upper = Math.floor(sum / 10);

nextList.next = new ListNode(val);

nextList = nextList.next;

l1 = l1 ? l1.next : null;

l2 = l2 ? l2.next : null;

}

// 注意考虑最后进位的情况

// 如果是 [5], [5]，则和为 10，即 0 -> 1

if (upper > 0) {

nextList.next = new ListNode(upper);

}

return l.next;

};