class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
     int ca = 0;
        ListNode head1 = l1;
        ListNode head2 = l2;
        ListNode dummy = new ListNode(-1);
        ListNode dummyHead = dummy;
        while(head1 != null || head2 != null){
            int n1 = head1 == null ? 0 : head1.val;
            int n2 = head2 == null ? 0 : head2.val;
            int res = n1 + n2 + ca;
            ListNode tmp = new ListNode(res % 10);
            dummyHead.next = tmp;
            dummyHead = tmp;
            if(res > 9){
                ca = 1;
            }else{
                ca = 0;
            }
            head1 = head1 == null ? null : head1.next;
            head2 = head2 == null ? null : head2.next;
        }

        if(ca == 1){
            ListNode tmp = new ListNode(1);
            dummyHead.next = tmp;
            dummyHead = tmp;
        }
        return dummy.next;
    }
}