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public class DLinkedList<T>
{
private Node firstNode;
private Node lastNode;
private Node middleNode;
private int numberOfEntries;
private int middlePosition;
public DLinkedList()
{
firstNode = null;
lastNode = null;
middleNode = null;
numberOfEntries = 0;
middlePosition = 0;
}
public void add(T newEntry)
{
if(firstNode == null)
{
firstNode = new Node(null, newEntry, null);
lastNode = firstNode;
}
else
{
Node newNode = new Node(lastNode, newEntry, null);
lastNode.next = newNode;
lastNode = newNode;
}
numberOfEntries++;
if(numberOfEntries % 2 == 1)
{
if(middleNode == null)
{
middleNode = firstNode;
}
else
{
middleNode = middleNode.next;
}
middlePosition++;
}
}
public T getEntry(int givenPosition)
{
T result = null;
if((givenPosition >= 1) && (givenPosition <= numberOfEntries))
{
result = (getNodeAt(givenPosition)).data;
}
return result;
}
private Node getNodeAt(int givenPosition)
{
Node currentNode = firstNode;
for(int counter = 1; counter < givenPosition; counter++)
{
currentNode = currentNode.next;
}
return currentNode;
}
public T getEntry1(int givenPosition)
{
T result = null;
if((givenPosition >= 1) && (givenPosition <= numberOfEntries))
{
result = (getNodeAt1(givenPosition)).data;
}
return result;
}
/**
* Modify this method according to the Solution 1
*/
private Node getNodeAt1(int givenPosition)
{
if(givenPosition > middlePosition)
{
//start from end, go backwars
Node temp = lastNode;
for(int i = 0;i<numberOfEntries - givenPosition;i++)
{
temp = temp.previous;
}
return temp;
}
else
{
//start from front, forwards
Node temp = firstNode;
for(int i = 1; i < givenPosition;i++)
{
temp = temp.next;
}
return temp;
}
}
public T getEntry2(int givenPosition)
{
T result = null;
if((givenPosition >= 1) && (givenPosition <= numberOfEntries))
{
result = (getNodeAt2(givenPosition)).data;
}
return result;
}
/**
* Modify this method according to the Solution 2
*/
private Node getNodeAt2(int givenPosition)
{
if(givenPosition < middlePosition)
{
if(givenPosition < middlePosition/2)
{
//STart from the front, go forwards
Node temp = firstNode;
for(int i = 1;i < givenPosition;i++)
{
temp = temp.next;
}
return temp;
}
else
{
//Start from the middle, backwards
Node temp = middleNode;
for(int i = middlePosition; i > givenPosition;i--)
{
temp = temp.previous;
}
return temp;
}
}
else
{
if(givenPosition < numberOfEntries*(3.0/4))
{
//Start from the middle, go forwards
Node temp = middleNode;
for(int i = middlePosition; i < givenPosition; i++)
{
temp = temp.next;
}
return temp;
}
else
{
//Start from the end, go backwards
Node temp = lastNode;
for(int i = numberOfEntries; i >givenPosition; i--)
{
temp = temp.previous;
}
return temp;
}
}
}
private class Node
{
private Node previous;
private T data;
private Node next;
private Node(Node previousNode, T aData, Node nextNode)
{
previous = previousNode;
data = aData;
next = nextNode;
}
}
}
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