2. Add Two Numbers

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 contain a single
digit. Add the two numbers and return it as a linked list.

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

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    int carry = 0;
    ListNode sentinel = new ListNode(0);
    ListNode curr = sentinel;
    while(l1 != null || l2 != null) {
        int x = l1 == null ? 0 : l1.val;
        int y = l2 == null ? 0 : l2.val;
        int sum = x + y + carry;
        carry = sum / 10;
        curr.next = new ListNode(sum % 10);
        curr = curr.next;
        l1 = l1 == null ? null : l1.next;
        l2 = l2 == null ? null : l2.next;
    }
    if(carry != 0) curr.next = new ListNode(1);
    return sentinel.next;
}