Question

Lets say you have 2 classes:

 - Node
 - Doubly linked list (DLL)

Your main creates a bunch of nodes, who know something about themselves, like their name for example and then calls add(Node*) method of the DLL.

In order to keep track of these Node pointers, (i would think) DLL should maintain an array of them. Is there a way to avoid it? (or any other data structure for that matter)

How is DLL implemented under the hood? Does it use arrays or something else?

Answer 1

There is a way to avoid it. You in fact don’t need to keep track of all the pointers in the one spot at all, because the Nodes themselves ARE the data structure.

EDIT: Just saw your Objective-C comment.

The Node should have (Objective-C syntax):

// the actual info at this list index
@property (retain) id ptr;

@property (retain) Node *previous;
@property (retain) Node *next;

And then your DLL class is simply a class with:

@property (retain) Node *first;
@property (retain) Node *last;
@property (assign) NSUInteger count;

That’s all you need. Inserting or deleting a Node involves pointer shuffling and a count adjustment, and access is sequential from either end.

add:(Node*)newNode would literally be:

[newNode setPrevious:[self last]];
[[self last] setNext:newNode];
[self setLast:newNode];
[self setCount:[self count]+1];

..whereas add:(Node*)newNode atIndex:(NSUInteger)index would be a little more complex:

if(index > self.count)
{
    // maybe throw an exception or something
} else if(index == self.count) // just do the normal add
{
    [self add:newNode];
    return;
} else if(index == 0) { // front-insert is also easier
    [newNode setNext:[self first]];
    [[self first] setPrevious:newNode];
    [self setFirst:newNode];
    [self setCount:[self count]+1];
    return;
}

// maybe do an extra check to see if it's quicker to search from the end
Node *currentLocation = [self first];
NSUInteger idx;
for(idx = 0; i < (index - 1); ++idx)
{
    currentLocation = [currentLocation next];
}

Node *previousNode = currentLocation; // store a reference to the previous node
currentLocation = [currentLocation next]; // now at the point we want to insert

[previousNode setNext:newNode];
[newNode setPrevious:previousNode];
[newNode setNext:currentLocation];
[currentLocation setPrevious:newNode];

[self setCount:[self count]+1];