#2. Add Two Numbers [Medium][Javascript]

문제

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.

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var addTwoNumbers = function(l1, l2) {
    let upNumber = 0;
    const head = new ListNode(0, null);
    let pr = head;

    while(l1 !== null || l2 !== null) { 
        const sum = (l1 ? l1.val : 0) + (l2 ? l2.val: 0);
        const current = new ListNode((sum + upNumber) % 10, null);
        upNumber = Math.floor((sum + upNumber) / 10);
        l1 = l1 ? l1.next : null;
        l2 = l2 ? l2.next : null;
        pr.next = current;
        pr = pr.next;
    }

    if(upNumber > 0) {
        pr.next = new ListNode(upNumber, null)
    }


    return head.next;
};