2. Add Two Numbers

You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode res = new ListNode(-1);
        ListNode tail = res;
        int carry = 0;
        while(l1 != null || l2 != null || carry != 0){
            int x1 = l1 == null ? 0 : l1.val;
            int x2 = l2 == null ? 0 : l2.val;
            tail.next = new ListNode((x1+x2+carry)%10);
            carry = (x1+x2+carry)/10;
            tail = tail.next;
            if(l1 != null) l1 = l1.next;
            if(l2 != null) l2 = l2.next;
        }
        return res.next;
    }
}