public class DoubleChain {

/* private DNode(double val) {

this(null, val, null);

}

private DNode(DNode prev, double val, DNode next) {

this.prev = prev;

this.val = val;

this.next =next;

}

*/

private DNode head;

public DoubleChain(double val) {

/* your code here. */

head = new DNode(val);

}

public DNode getFront() {

return head;

}

/** Returns the last item in the DoubleChain. */

public DNode getBack() {

/* your code here */

DNode p = head;

while (p.next != null) {

p = p.next;

} // close while loop

return p;

}

/** Adds D to the front of the DoubleChain. */

public void insertFront(double d) {

/* your code here */

DNode oldHead = head;

head = new DNode(oldHead.prev, d, oldHead);

}

/** Adds D to the back of the DoubleChain. */

public void insertBack(double d) {

/* your code here */

DNode p = head;

while (p.next != null) {

p = p.next;

} // close while

// DNode oldBack = p;

p.next = new DNode(p, d, null);

}

/** Removes the last item in the DoubleChain and returns it.

* This is an extra challenge problem. */

public DNode deleteBack() {

/* your code here */

DNode p = head;

while (p.next != null) {

p = p.next;

} // close while

p = p.prev;

return p;

}

/** Returns a string representation of the DoubleChain.

* This is an extra challenge problem. */

public String toString() {

/* your code here */

// present front-to-back chain

String chain = "The double chain is: ";

String arrow = "->";

DNode p = head;

while (p.next != null) {

chain = chain + p.val + arrow;

p = p.next;

}

chain += "null;";

return chain;

}

/* DNode */

public static class DNode {

public DNode prev;

public DNode next;

public double val;

private DNode(double val) {

this(null, val, null);

}

private DNode(DNode prev, double val, DNode next) {

this.prev = prev;

this.val = val;

this.next =next;

}

}

public static void main(String[] args) {

DoubleChain L = new DoubleChain(0);

L.insertFront(1);

L.insertFront(2);

L.insertFront(3);

System.out.println(L.toString());

}

}