Linked List methods

Test.java

import java.util.ArrayList;
import java.util.List;

class Test {

static class ListNode {
    int data;
    ListNode next;
    public ListNode(int data) {
      this.data = data;
    }
    public String toString() {
      return this.data+"";
    }
  }

public static void main(String[] args)
 {
    ListNode n1 = new ListNode(1);
    ListNode n2 = new ListNode(2);
    n1.next = n2;
    ListNode n3 = new ListNode(3);
    n2.next = n3;
    ListNode n4 = new ListNode(4);
    n3.next = n4;
    ListNode n5 = new ListNode(5);
    n4.next = n5;
    //ListNode rotated  = rotate(n1);
    reverseN(n1,3);
 }

 public static ListNode reverseN(ListNode head, int n) {
  ListNode prev = null, next = null;
  List<ListNode> lists = new ArrayList<>();
  int counter = 0;
  ListNode curr = head;
  ListNode lastHead = null;
  while(curr != null) {
    if(counter == n) {
      if(lastHead != null) {
        lastHead.next = prev;
      }
      lastHead = head;
      head = curr;
      lists.add(prev);
      prev = null;
      counter = 0;
    }
    next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
    counter++;
  }

  if(counter > 0) {
    lists.add(prev);
    lastHead.next = prev;
  }

  System.out.println(lists);
  printNode(lists.get(0));
  return null;
 }

 public static ListNode rotate(ListNode head) {
  ListNode node = head;
  ListNode center = findCenter(node);
  ListNode secondHalf = reverse(center);
  printNode(secondHalf);
  printNode(head);
  ListNode ret = new ListNode(-1);
  ListNode curr = ret;
  int i = 0;
  while(head != null) {
    if(i%2 == 0) {
      curr.next = head;
      head = head.next;
    } else {
      curr.next = secondHalf;
      secondHalf = secondHalf.next;
    }
    i++;
    curr = curr.next;
  }
  printNode(ret.next);
  return ret.next;
 }

 private static ListNode reverse(ListNode head) {
  ListNode curr = head;
  ListNode prev = null, next = null;
  while (curr != null) {
    next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
  }
  return prev;
 }

 private static ListNode findCenter(ListNode head) {
  ListNode fp = head;
  ListNode sp = head;
  while (fp != null) {
    fp = fp.next;
    if(fp != null && fp.next != null) {
      fp = fp.next;
      sp = sp.next;
    }
  }
  return sp;
 }

 private static void printNode(ListNode head) {
   while (head != null) {
      System.out.print(head.data);
      head = head.next;
   }
   System.out.println();
 }



}