/* 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;

}

}