8 3/11: Skip lists

Imperative lists

If you remember back to lecture, we’ve started looking at new ways of building data structures that use mutation. The main goal of this lab will be to develop a new kind of data structure, but first we will need to define a new kind of list that is mutable. (It will come in handy later.)

Here’s an interface (in comments) for a mutable list, which lets you modify a list with the set method. The set method takes an index and an element and destructively updates the list to contain the given element at the given index. We’ve also included a ref method that produces the element at a given index:

// A List<V> implements


// ref : Integer -> V

// returns the element at the given index


// set : Integer V -> Void

// Effect: updates the element in-place

// sets the element at the given index

Exercise 1. Write down a data definition that implements the above interface.

Exercise 2. Implement the data defintion you wrote down and implement both of the ref and set methods.

Another day, another data structure

There are many ways for storing a sorted collection of data, such as sorted lists and balanced search trees. Both of these implement the interface of an ordered (or sorted) set. However, both of these data structures have their own issues.

With sorted lists, insertion and search of an element of the set takes time proportional to the entire length of the list in the worst case since you can end up traversing the whole list.

Binary search trees are much better in that these operations are both logarithmic in the number of elements. On the other hand, they are more complicated than lists and can take up a significant amount of memory on your computer.

Could we do even better? It turns out you can, for some definition of “better”. We will look at a data structure called a skip list that uses randomization to provide the same expected running time as a binary search tree, but using a data structure that is simpler and potentially uses less memory ("expected" here means that over a large number of trials we will get that running time on average).

Building skip lists

A skip list is essentially a stack of linked lists that are constructed in a specific way. The stack has some fixed maximum level (let’s say 5 for this lab).

Each element in the skip list will participate in some number of the linked lists (up to the max level). For each element you want to add, you flip a coin for each level and the number of heads tells you how many of the lists this element will be included in. The effect of this is that elements are less likely to show up in higher levels of the skip list and more likely to show up at the bottom. The one exception is that all elements are in the bottom list, so that the bottom list is a normal linked list.

Here is a picture from Wikipedia of what a skip list might look like:

We will implement a variant of a skip list that is never empty, so the constructor takes an initial key and value to start out with. This makes the implementation a little bit simpler (there is no "header" from that picture). The initial element will show up at all levels.

// A SkipList<V> is a

//   new SkipList<V>(Integer, V)

// and implements


// insert : Integer V -> Void

// Effect: updates the list with a new element

// add an element with the given integer key


// search : Integer -> V

// look up the element matching the given key

The new SkipList<V> (n ,v) expression creates a new skip list with a single element. Notice that the contract for insert states that the result is Void. We are going to implement this skip list using mutation because it is a data structure that is easier to implement in this style.

To implement the skip list, we will use an auxillary data definition.

// A Node<V> is one of

//   new MTNode()

//   new Element(Integer, V, Integer)

// and implements


// isEmpty : -> Boolean

// specify whether this is the end


// getKey : -> Integer

// get the key at this node


// getValue : -> V

// get the value at this node


// setValue : Integer V -> Void

// Effect: updates the value in-place

// sets the value for this node


// getNext : Integer -> Node<V>

// gets the next node linked at the given level


// setNext : Integer Node<V> -> Void

// Effect: updates the next node in-place

// sets the next node linked at the given level

A Node is a data definition and interface for the elements in the list. The end of the list is marked by MTNode while the elements in between are Elements. Since MTNode shouldn’t do anything, you can just implement stubs for its methods or return errors.

For each Element, you should use the List implementation that you defined earlier to track the nodes linked to from this node. There will be a link for every level that this node is contained in. Its constructor takes the initial key and value (Integer and V types respectively) and also the height of this node.

The algorithm for the search method is easier, so we will start with that.

To search a skip list, start at the very top level and keep going right. When you either hit the end (an MTNode) or you hit a key that is bigger than the one you’re looking for, you go down a level. It will probably be helpful to write this as a helper function that recurs on the level of the skip list.

For example, if you are looking for the key 4 in the example skip list in the picture above, you would start at the top and immediately see that you end up at the end. Then you start at the second level from the top and find a node with 4, so you’re done.

If you were looking for 5, you would go to 4 and then find that the next node is 6, so you go down again. Since the next node is still 6, you will go down again. Finally, the next node is 5 at the bottom row so you’re done (now you might see why it’s called a "skip" list).

Exercise 3. Implement the search method and test it with a one-element skip list.

For insertion, the code is very similar. However, this time you have to keep track of an extra thing. Every time you go down a level while searching, you have to record the links that you need to update when you add the new node.

If the node with the key already exists in the tree, just find it and then update its value.

If the node does not yet exist, use the same technique you used to search through the tree, but keep storing the right-most node that you looked at in a list. Then build a new node. For the new node, you will want to randomly flip a coin up to maxLevel - 1 (so 4) times. When you see the first tails, the number of heads so far plus one is the height of the new node (this means that taller nodes are less likely).

At the very end, use setNext to set the next node for the new node to the next nodes for the nodes you kept track of in your list. Then update their next elements to be the new node that you are adding into the list. You only need to update the next links up to the height of the new node.

Exercise 4. Implement the insert method. Now you can write better tests for the search method using insert to build bigger skip lists.

Now that you’ve implemented the basic functionality, try to think of how you would implement a delete method. First of all, what should its contract be?

Exercise 5. Write down the changes you need to your interface to support delete.

Now implement the method. Its structure should be very similar to insert.