LeetCode 2: Add Two Numbers in Java

Each linked list stores a non-negative integer in reverse digit order. Add corresponding nodes and return the sum in the same representation.

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        int carry = 0;

        while (l1 != null || l2 != null || carry != 0) {
            int x = l1 == null ? 0 : l1.val;
            int y = l2 == null ? 0 : l2.val;
            int sum = x + y + carry;

            carry = sum / 10;
            tail.next = new ListNode(sum % 10);
            tail = tail.next;

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

        return dummy.next;
    }
}

The loop continues while either list has nodes or a carry remains. Missing digits are treated as zero. For 2 -> 4 -> 3 plus 5 -> 6 -> 4, the result is 7 -> 0 -> 8.

Time complexity is O(max(m, n)); the returned list uses the same order of space.