Remove Nth Node From End of List

public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        if(head == null || n <= 0)    return head;
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        int count = 0;
        HashMap<Integer,ListNode> hm = new HashMap<Integer,ListNode>();
        hm.put(count++,dummy);
        while(head != null)
        {
            hm.put(count++,head);
            head = head.next;
        }
        int last = count - 1 - n;
        hm.get(last).next = hm.get(last).next.next;
        return dummy.next;
    }
}

   public ListNode removeNthFromEnd(ListNode head, int n) {

    ListNode start = new ListNode(0);
    ListNode slow = start, fast = start;
    slow.next = head;

    //Move fast in front so that the gap between slow and fast becomes n
    for(int i=1; i<=n+1; i++)   {
        fast = fast.next;
    }
    //Move fast to the end, maintaining the gap
    while(fast != null) {
        slow = slow.next;
        fast = fast.next;
    }
    //Skip the desired node
    slow.next = slow.next.next;
    return start.next;
}