## Problem

You have two numbers represented by a linked list, where each node contains a single digit. The digits are stored in reverse order, such that the 1's digit is at the head of the list. Write a function that adds the two numbers and returns the sum as a linked list.

## Example

Given 7->1->6 + 5->9->2. That is, 617 + 295.

Return 2->1->9. That is 912.

Given 3->1->5 and 5->9->2, return 8->0->8.

## Code - Java

/**
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
/**
* @param l1: the first list
* @param l2: the second list
* @return: the sum list of l1 and l2
*/
public ListNode addLists(ListNode l1, ListNode l2) {
int carry = 0;
while (l1 != null || l2 != null) {
if (l1 == null) {
ptr.next = new ListNode((l2.val + carry) % 10);
carry = (l2.val + carry) / 10;
l2 = l2.next;
} else if (l2 == null) {
ptr.next = new ListNode((l1.val + carry) % 10);
carry = (l1.val + carry) / 10;
l1 = l1.next;
} else {
ptr.next = new ListNode((l1.val + l2.val + carry) % 10);
carry = (l1.val + l2.val + carry) / 10;
l1 = l1.next;
l2 = l2.next;
}
ptr = ptr.next;
}
if (carry == 1) {
ptr.next = new ListNode(1);
}
}