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 -> 8Explanation: 342 + 465 = 807 解题思路： （1）本题涉及到链表的结构 （2）本题类似小学数学的加法，从个位开始相加，有超过10则进位，否则就是本节点的值 （3）两个链表的长度不知道谁长，需要做一下处理。 code：

/** * 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 list = new ListNode(0);    var result = list;    var sum, carry = 0;    while(l1 || l2 || carry>0){  //当l1 或 l2 或有进位存在的时候，加法计算就没有结束        sum=0;        if(l1!==null){            sum += l1.val;            l1=l1.next;        }        if(l2!==null){            sum += l2.val;            l2=l2.next;        }        sum += carry;        list.next = new ListNode(sum%10);        carry = parseInt(sum/10);        list = list.next;    }    return result.next;    };