class ListNode { val: number next: ListNode | null

constructor(val?: number, next?: ListNode | null) {
    this.val = (val === undefined ? 0 : val)
    this.next = (next === undefined ? null : next)
}

}

function addTwoNumbers(l1: ListNode | null, l2: ListNode | null): ListNode | null { let dummy = new ListNode(0); let current = dummy; let carry = 0;

while (l1 !== null || l2 !== null || carry !== 0) {
    const val1 = l1 ? l1.val : 0;
    const val2 = l2 ? l2.val : 0;

    const sum = val1 + val2 + carry;
    carry = Math.floor(sum / 10);

    current.next = new ListNode(sum % 10);
    current = current.next;

    if (l1) l1 = l1.next;
    if (l2) l2 = l2.next;
}