ReorderList.java

class ReorderList {
    public void reorderList(ListNode head) {
        if (head == null || head.next == null) {
            return;
        }
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode curr = head;
        ListNode revCurr = reverse(slow.next);
        slow.next = null;
        while (curr != null && revCurr != null) {
            ListNode next = curr.next;
            ListNode revNext = revCurr.next;
            curr.next = revCurr;
            curr = next;
            revCurr.next = next;
            revCurr = revNext;
        }
    }
    private ListNode reverse(ListNode head) {
        ListNode curr = head;
        ListNode prev = null;
        while (curr != null) {
            ListNode next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
}