class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode h1 = l1, h2 = l2, dummy = new ListNode(-1, h1), d = dummy;
        int carry = 0, sum = 0;
        
        while(h1 != null && h2 != null){
            sum = h1.val + h2.val + carry;
            carry = sum / 10;
            h1.val = sum % 10;
            d.next = h1;

            d = d.next;
            h1 = h1.next;
            h2 = h2.next;
        }
        while(h1 != null){
            sum = h1.val + carry;
            carry = sum / 10;
            h1.val = sum % 10;
            d.next = h1;

            d = d.next;
            h1 = h1.next;
        }
        while(h2 != null){
            sum = h2.val + carry;
            carry = sum / 10;
            h2.val = sum % 10;
            d.next = h2;

            d = d.next;
            h2 = h2.next;
        }
        if(carry != 0){
            d.next = new ListNode(carry);
        }

        return dummy.next;
    }
}

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        return addTwoNumbers(l1, l2, 0);
    }

    ListNode addTwoNumbers(ListNode l1, ListNode l2, int carry) {
        if(l1 == null && l2 == null){
            if(carry != 0) return new ListNode(carry);
            else return null;
        }
        if(l1 == null) {
            int sum = l2.val + carry;
            l2.val = sum % 10;
            carry = sum / 10;
            l2.next = addTwoNumbers(null, l2.next, carry);
            return l2;
        }
        if(l2 == null){
            int sum = l1.val + carry;
            l1.val = sum % 10;
            carry = sum / 10;
            l1.next = addTwoNumbers(l1.next, l2, carry);
            return l1;
        }

        int sum = l1.val + l2.val + carry;
        l1.val = sum % 10;
        carry = sum / 10;
        l1.next = addTwoNumbers(l1.next, l2.next, carry);
        return l1;
    }
}