LeetCode - Add Two Numbers

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int index = 0;
        int overflow = 0;
        ListNode returnList = null;
        ListNode linkedListPtr = null;

        while (l1 != null && l2 != null) {
            int one = l1.val;
            int two = l2.val;

            int sum = one + two + overflow;
            // Calculate overflow
            overflow = sum / 10;
            sum = sum % 10;

            ListNode curNode = new ListNode(sum);

            if (returnList == null) {
                returnList = curNode;
                linkedListPtr = returnList;
            }
            else {
                linkedListPtr.next = curNode;
                linkedListPtr = linkedListPtr.next;
            }

            l1 = l1.next;
            l2 = l2.next;
        }

        // Add any remaining numbers in l1 and l2
        while (l1 != null) {
            int sum = l1.val + overflow;
            // Calculate overflow
            overflow = sum / 10;
            sum = sum % 10;
            ListNode curNode = new ListNode(sum);
            linkedListPtr.next = curNode;
            linkedListPtr = linkedListPtr.next;
            l1 = l1.next;
        }

        while (l2 != null) {
            int sum = l2.val + overflow;
            // Calculate overflow
            overflow = sum / 10;
            sum = sum % 10;
            ListNode curNode = new ListNode(sum);
            linkedListPtr.next = curNode;
            linkedListPtr = linkedListPtr.next;
            l2 = l2.next;
        }

        // IF overflow is still not 0
        if (overflow != 0) {
            ListNode curNode = new ListNode(overflow);
            linkedListPtr.next = curNode;
            linkedListPtr = linkedListPtr.next;

            overflow = 0;
        }

        return returnList;
    }
}