AddTwoNumbers.java

public class AddTwoNumbers {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode output = dummy;

        int carry = 0;
        while (l1 != null || l2 != null) {
            int a = l1 != null ? l1.val : 0;
            int b = l2 != null ? l2.val : 0;

            int sum = a + b + carry;
            carry = sum / 10;
            int lastDigit = sum % 10;
            ListNode newNode = new ListNode(lastDigit);
            output.next = newNode;
            if (l1 != null)
                l1 = l1.next;
            if (l2 != null)
                l2 = l2.next;
            output = output.next;
        }
        if (carry > 0) {
            ListNode newNode = new ListNode(carry);
            output.next = newNode;
            output = output.next;
        }
        return dummy.next;
    }
}

class ListNode {
    int val;
    ListNode next;

    ListNode() {
    }

    ListNode(int val) {
        this.val = val;
    }

    ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}