/* remember to use carry */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        ListNode dummy = new ListNode(-1); */ head node not certain, use dummy node */
        ListNode head = dummy;
        int sum = 0, carry = 0;
        while(l1 != null && l2 != null){
            sum = l1.val + l2.val + carry;
            carry = sum / 10;
            head.next = new ListNode(sum % 10);
            l1 = l1.next;
            l2 = l2.next;
            head = head.next;
        }
        while(l1 != null){
            sum = l1.val + carry;
            carry = sum / 10;
            head.next = new ListNode(sum % 10);
            l1 = l1.next;
            head = head.next;
        }

        while(l2 != null){
            sum = l2.val + carry;
            carry = sum / 10;
            head.next = new ListNode(sum % 10);
            l2 = l2.next;
            head = head.next;
        }
        if(carry != 0) head.next = new ListNode(carry);
        return dummy.next;
    }
}