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.

    /**
     * @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) {
        // write your code here
        ListNode head = new ListNode(0);
        ListNode current = head;
        int carry = 0;
        while (l1 != null || l2 != null || carry != 0){
            if(l1 != null) {
                carry += l1.val;
                l1 = l1.next;
            }
            if(l2 != null){
                carry += l2.val;
                l2 = l2.next;
            }
            current.next = new ListNode(carry % 10);
            current = current.next;
            carry /= 10;
        }
        return head.next;
    }
}

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """

        if not l1 or not l2:
            return l1 or l2

        carry = 0
        curr = dummy = ListNode(0)
        while l1 or l2 or carry:
            if l1:
                carry += l1.val
                l1 = l1.next
            if l2:
                carry += l2.val
                l2 = l2.next
            curr.next = ListNode(carry % 10)
            curr = curr.next
            carry //= 10

        return dummy.next