import java.util.*;

/**
 * Name: XXXXXX StudentID: XXXXXXX
 * Doubly-linked list implementation of a <tt>List12</em>.
 */
class DoublyLinkedList12 implements List12 {
        private Node _head, _tail;
        private int _size;

	// Inner class to represent the "nodes" of the linked list
        private static class Node {
                private Node _next, _prev;
                private Object _data;
        }

	// Other methods...
}