Add Two Numbers

题目：

You are given two linked lists representing two non-negative numbers. The digits are
stored in reverse order and each of their nodes contains a single digit. Add the two
numbers and return it as a linked list.
Input: (2 -> 4 -> 3) + (5 ->  6 ->  4)
Output: 7 ->  0 ->  8

解答：

public ListNode  addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode dummyHead = new ListNode(0);
    ListNode p = l1;
    ListNode q = l2;
    ListNode curr = dummyHead;

    int carry = 0;

    while(p != null || q != null) {
        int x = (p != null)?p.val:0;
        int y = (q != null)?q.val:0;

        int digit = x + y + carry;
        carry = digit / 10;
        curr.next = new ListNode(digit % 10);
        curr = curr.next;

        if(p != null) {
            p = p.next;
        }

        if(q != null) {
            q = q.next;
        }
    }

    if(carry > 0) {
        curr.next = new ListNode(carry);
    }

    return dummyHead;
}