ReorderLL.java

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

    public void reorderList(ListNode head) {
        //middle 
        ListNode slow=head;
        ListNode fast=head;
        while(fast!=null && fast.next!=null){
            slow=slow.next;
            fast=fast.next.next;
        }
        //reverse 
        ListNode prev=null;
        ListNode curr=slow.next;
        slow.next=null;
        while(curr!=null){
            ListNode next=curr.next;
            curr.next=prev;
            prev=curr;
            curr=next;
        }
        //merge
        ListNode first=head;
        ListNode second=prev;
        while(second!=null){
            ListNode temp1=first.next;
            ListNode temp2=second.next;
            first.next=second;
            second.next=temp1;
            first=temp1;
            second=temp2;

        }
    }

}