Add Two Numbers

Problem Description

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 contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]
Output: [0]

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

Constraints:

• The number of nodes in each linked list is in the range [1, 100].
• 0 <= Node.val <= 9
• It is guaranteed that the list represents a number that does not have leading zeros.

Solution (JavaScript)

/**
 * 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 head = new ListNode(0);
    var tail = head;
    var carryover = 0;
    
    while(l1 != null || l2 != null){
        var sum = (l1 == null ? 0 : l1.val) 
                  + (l2 == null ? 0 : l2.val) 
                  + carryover;
        carryover = Math.floor(sum / 10);
        var nextNode = new ListNode(sum % 10);
        tail.next = nextNode;
        tail = tail.next;
        l1 = l1 ? l1.next : null;
        l2 = l2 ? l2.next : null;
    }
    
    if(carryover > 0){
        var nextNode = new ListNode(carryover);
        tail.next = nextNode;
    }
    
    return head.next;
};