How do I shuffle nodes in a linked list? - data-structures

Question

I just started a project for my Java2 class and I've come to a complete stop. I just can't get
my head around this method. Especially when the assignment does NOT let us use any other DATA STRUCTURE or shuffle methods from java at all.
So I have a Deck.class in which I've already created a linked list containing 52 nodes that hold 52 cards.
public class Deck {
private Node theDeck;
private int numCards;
public Deck ()
{
while(numCards < 52)
{
theDeck = new Node (new Card(numCards), theDeck);
numCards++;
}
}
public void shuffleDeck()
{
int rNum;
int count = 0;
Node current = theDeck;
Card tCard;
int range = 0;
while(count != 51)
{
// Store whatever is inside the current node in a temp variable
tCard = current.getItem();
// Generate a random number between 0 -51
rNum = (int)(Math.random()* 51);
// Send current on a loop a random amount of times
for (int i=0; i < rNum; i ++)
current = current.getNext(); ******<-- (Btw this is the line I'm getting my error, i sort of know why but idk how to stop it.)
// So wherever current landed get that item stored in that node and store it in the first on
theDeck.setItem(current.getItem());
// Now make use of the temp variable at the beginning and store it where current landed
current.setItem(tCard);
// Send current back to the beginning of the deck
current = theDeck;
// I've created a counter for another loop i want to do
count++;
// Send current a "count" amount of times for a loop so that it doesn't shuffle the cards that have been already shuffled.
for(int i=0; i<count; i++)
current = current.getNext(); ****<-- Not to sure about this last loop because if i don't shuffle the cards that i've already shuffled it will not count as a legitimate shuffle? i think? ****Also this is where i sometimes get a nullpointerexception****
}
}
}
Now I get different kinds of errors
When I call on this method:
it will sometimes shuffle just 2 cards but at times it will shuffle 3 - 5 cards then give me a NullPointerException.
I've pointed out where it gives me this error with asterisks in my code above
at one point I got it to shuffle 13 cards but then everytime it did that it didn't quite shuffle them the right way. one card kept always repeating.
at another point I got all 52 cards to go through the while loop but again it repeated one card various times.
So I really need some input in what I'm doing wrong. Towards the end of my code I think my logic is completely wrong but I can't seem to figure out a way around it.

Answer 1

Seems pretty long-winded.
I'd go with something like the following:
public void shuffleDeck() {
for(int i=0; i<52; i++) {
int card = (int) (Math.random() * (52-i));
deck.addLast(deck.remove(card));
}
}
So each card just gets moved to the back of the deck in a random order.

Answer 2

If you are authorized to use a secondary data structure, one way is simply to compute a random number within the number of remaining cards, select that card, move it to the end of the secondary structure until empty, then replace your list with the secondary list.

Answer 3

My implementation shuffles a linked list using a divide-and-conquer algorithm
public class LinkedListShuffle
{
public static DataStructures.Linear.LinkedListNode<T> Shuffle<T>(DataStructures.Linear.LinkedListNode<T> firstNode) where T : IComparable<T>
{
if (firstNode == null)
throw new ArgumentNullException();
if (firstNode.Next == null)
return firstNode;
var middle = GetMiddle(firstNode);
var rightNode = middle.Next;
middle.Next = null;
var mergedResult = ShuffledMerge(Shuffle(firstNode), Shuffle(rightNode));
return mergedResult;
}
private static DataStructures.Linear.LinkedListNode<T> ShuffledMerge<T>(DataStructures.Linear.LinkedListNode<T> leftNode, DataStructures.Linear.LinkedListNode<T> rightNode) where T : IComparable<T>
{
var dummyHead = new DataStructures.Linear.LinkedListNode<T>();
DataStructures.Linear.LinkedListNode<T> curNode = dummyHead;
var rnd = new Random((int)DateTime.Now.Ticks);
while (leftNode != null || rightNode != null)
{
var rndRes = rnd.Next(0, 2);
if (rndRes == 0)
{
if (leftNode != null)
{
curNode.Next = leftNode;
leftNode = leftNode.Next;
}
else
{
curNode.Next = rightNode;
rightNode = rightNode.Next;
}
}
else
{
if (rightNode != null)
{
curNode.Next = rightNode;
rightNode = rightNode.Next;
}
else
{
curNode.Next = leftNode;
leftNode = leftNode.Next;
}
}
curNode = curNode.Next;
}
return dummyHead.Next;
}
private static DataStructures.Linear.LinkedListNode<T> GetMiddle<T>(DataStructures.Linear.LinkedListNode<T> firstNode) where T : IComparable<T>
{
if (firstNode.Next == null)
return firstNode;
DataStructures.Linear.LinkedListNode<T> fast, slow;
fast = slow = firstNode;
while (fast.Next != null && fast.Next.Next != null)
{
slow = slow.Next;
fast = fast.Next.Next;
}
return slow;
}
}

Answer 4

Just came across this and decided to post a more concise solution which allows you to specify how much shuffling you want to do.
For the purposes of the answer, you have a linked list containing PlayingCard objects;
LinkedList<PlayingCard> deck = new LinkedList<PlayingCard>();
And to shuffle them use something like this;
public void shuffle(Integer swaps) {
for (int i=0; i < swaps; i++) {
deck.add(deck.remove((int)(Math.random() * deck.size())));
}
}
The more swaps you do, the more randomised the list will be.