2. Add Two Numbers

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
Explanation: 342 + 465 = 807.

/**
 * 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 sm = 0, dummy = cur = new ListNode(0);
    while (l1 || l2 || sm){
        if (l1) {
            sm += l1.val;
            l1 = l1.next;
        } 
        if (l2) {
            sm += l2.val;
            l2 = l2.next;
        } 
        cur = cur.next = new ListNode(sm % 10);
        if (sm > 9) {
            sm = 1;
        } else{
            sm = 0;
        }
    }
    return dummy.next;
};