Grokking 223-1

class Solution {
    public ListNode reverse(ListNode root){
        ListNode cur = root;
        ListNode prev = null;
        while (cur != null) {
            ListNode temp = cur.next;
            cur.next = prev;
            prev = cur;
            cur = temp;
        }
        return prev;
    }

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode l1Reversed = reverse(l1);
        ListNode l2Reversed = reverse(l2);
        ListNode sumHead = new ListNode();
        ListNode curr = sumHead;
        int carry = 0;
        int sum = 0;
        while (l1Reversed != null || l2Reversed != null || carry != 0) {
            sum = carry;
            if (l1Reversed != null) {
                sum += l1Reversed.val;
                l1Reversed = l1Reversed.next;
            }
            if (l2Reversed != null) {
                sum += l2Reversed.val;
                l2Reversed = l2Reversed.next;
            }
            carry = sum / 10;
            sum = sum % 10;
            curr.next = new ListNode(sum);
            curr = curr.next;
        }

        ListNode ansReversed = sumHead.next;
        return reverse(ansReversed);
    }
}