//****************************  DLL.java  *******************************
//                  generic doubly linked list class

public class DLL<T> {
    private DLLNode<T> head, tail;
    public DLL() {
        head = tail = null;
    }
    public boolean isEmpty() {
        return head == null;
    }
    public void setToNull() {
        head = tail = null;
    }
    public T firstEl() {
        if (head != null)
             return head.info;
        else return null;
    }
    public void addToHead(T el) {
        if (head != null) {
             head = new DLLNode<T>(el,head,null);
             head.next.prev = head;
        }
        else head = tail = new DLLNode<T>(el);
    }
    public void addToTail(T el) {
        if (tail != null) {
             tail = new DLLNode<T>(el,null,tail);
             tail.prev.next = tail;
        }
        else head = tail = new DLLNode<T>(el);
    }
    public T deleteFromHead() {
        if (isEmpty()) 
             return null;
        T el = head.info;
        if (head == tail)   // if only one node on the list;
             head = tail = null;
        else {              // if more than one node in the list;
             head = head.next;
             head.prev = null;
        }
        return el;
    }
    public T deleteFromTail() {
        if (isEmpty()) 
             return null;
        T el = tail.info;
        if (head == tail)   // if only one node on the list;
             head = tail = null;
        else {              // if more than one node in the list;
             tail = tail.prev;
             tail.next = null;
        }
        return el;
    }
    public void printAll() { 
        for (DLLNode<T> tmp = head; tmp != null; tmp = tmp.next)
             System.out.print(tmp.info + " ");
    }
    public T find(T el) {
        DLLNode<T> tmp;
        for (tmp = head; tmp != null && !tmp.info.equals(el); tmp = tmp.next);
        if (tmp == null)
             return null;
        else return tmp.info;
    }
}