2.3

deleteMiddle.java

/**
 * Created by Readman on 6/23/14.
 */
public class deleteMiddle {
    /*
    time n
    space 1
    * */
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        deleteMiddle(head);
        while (head != null) {
            System.out.print(head.val +" ");
            head = head.next;
        }
    }
    public static void deleteMiddle (ListNode head) {
        ListNode slow = head;
        ListNode fast = head.next; // use next or record previous node of slow.
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        slow.next = slow.next.next;
    }
}

you misunderstood this problem's meaning. the input is the node which need to be deleted.

Never write something like (fast.next != null && fast.next.next != null) in the program. It will cause NPE in some cases.