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

use a variable sum to track the current add result, in the end, if sum is more than 0, add a new node in the end of the linkedlist.

//the result is also in reverse order
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1==null||l2==null) {
            return null;
        }

        ListNode previous = new ListNode(0);
        ListNode head = previous;
        int sum = 0;
        while(l1!=null||l2!=null) {
            if (l1!=null) {
                sum = sum + l1.val;
                l1 = l1.next;
            }
            if (l2!=null) {
                sum = sum + l2.val;
                l2 = l2.next;
            }
            head.next = new ListNode(sum%10);
            head = head.next;
            //carry on
            sum = sum / 10;
        }
        //{5},{5}
        if (sum==1) {
            head.next = new ListNode(1);
        }
        return previous.next;
    }