Reverse Second Half Linked List

Given a linked list, reverse the second half linked list. If the list length is odd, then the middle list node also should be reversed.

For example:
Given:  1 -> 2 -> 3 -> null;
Return: 1 -> 3 -> 2 -> null;

Given:  1 -> 2 -> 3 -> 4 -> null;
Return: 1 -> 2 -> 4 -> 3 -> null;

/*
First compute the size of list to determine the middle list node.

time:O(n), space:O(1)
*/

import java.util.*;

class ListNode {
    int val;
    ListNode next;
    public ListNode(int val) {
        this.val = val;
        next = null;
    }
}

public class Solution {
    public static ListNode reverseHalfList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        int size = calSize(head);
        ListNode middle = getMidNode(head, size);
        middle.next = reverse(middle.next);
        return head;
    }
    public static int calSize(ListNode head) {
        int size = 0;
        while (head != null) {
            head = head.next;
            size++;
        }
        return size;
    }
    public static ListNode getMidNode(ListNode head, int size) {
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode slow = (size & 1) == 0 ? head : dummy;
        ListNode fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    public static ListNode reverse(ListNode head) {
        ListNode newHead = null;
        while (head != null) {
            ListNode temp = head.next;
            head.next = newHead;
            newHead = head;
            head = temp;
        }
        return newHead;
    }

    public static void main(String[] args) {
        ListNode n1 = new ListNode (1);
        ListNode n2 = new ListNode (2);
        ListNode n3 = new ListNode (3);
        ListNode n4 = new ListNode (4);
        ListNode n5 = new ListNode (5);
        ListNode n6 = new ListNode (6);
        ListNode n7 = new ListNode (7);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        
        n5.next = n6;
        n6.next = n7;
        ListNode ans1 = reverseHalfList(n1);
        ListNode ans2 = reverseHalfList(n5);
        while (ans1 != null) {
            System.out.print(ans1.val + " ");
            ans1 = ans1.next;
        }
        System.out.println(" ");
        while (ans2 != null) {
            System.out.print(ans2.val + " ");
            ans2 = ans2.next;
        }
    }
}