Reorder_List.java

package com.leet_code;

public class Reorder_List {// didn't understand properly
    public class ListNode{
        int val;
        ListNode next;
        ListNode(){}
    }
    public void reorderList(ListNode head) {
        if (head == null || head.next == null) {
            return;
        }
        ListNode mid = middleNode(head);
        ListNode hs = reverseList(mid);
        ListNode hf = head;
        while (hf != null && hs != null) {
            ListNode temp = hf.next;
            hf.next = hs;
            hf = temp;

            temp = hs.next;
            hs.next = hf;
            hs = temp;
        }
        if (hf != null) {
            hf.next = null;
        }}
    public ListNode middleNode(ListNode head) {
        ListNode s = head;
        ListNode f = head;

        while (f != null && f.next != null) {
            s = s.next;
            f = f.next.next;
        }
        return s;
    }
    public ListNode reverseList(ListNode head) {
        if (head == null) {
            return head;
        }
        ListNode prev = null;
        ListNode present = head;
        ListNode next = present.next;

        while (present != null) {
            present.next = prev;
            prev = present;
            present = next;
            if (next != null) {
                next = next.next;
            }
        }
        return prev;
    }
}