You are given two linked lists representing two non-negative numbers. 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.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Solution
注意进位

/**
 * 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 sum = l1.val + l2.val;
    var l3 = new ListNode(sum % 10);
    var node = l3;
    var next1 = l1.next;
    var next2 = l2.next;
    sum = Math.floor(sum/10);
    while(next1 || next2 || sum) {
        sum += (next1 ? next1.val : 0) + (next2 ? next2.val : 0);
        node.next = new ListNode(sum % 10);
        sum = Math.floor(sum/10);
        next1 = next1 && next1.next;
        next2 = next2 && next2.next;
        node = node.next;
    }
    return l3;
};