reorder list

public void (ListNode head) {
    if (head == null || head.next == null)
        return;
    
    ListNode slow = head, fast = head.next;
    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;
    }
    ListNode ptr = slow.next;
    slow.next = null;
    // reverse the second part
    ListNode prev = null;
    while (ptr != null) {
        ListNode temp = ptr.next;
        ptr.next = prev;
        prev = ptr;
        ptr = temp;
    }
    // combine the two
    ListNode first = head, second = prev;
    while (first != null && second != null) {
        ListNode firstNext = first.next;
        ListNode secondNext = second.next;
        first.next = second;
        second.next = firstNext;
        first = firstNext;
        second = secondNext;
    }
}