# 4.1: The Basic Structure

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Conceptually, a skiplist is a sequence of singly-linked lists $$L_0,\ldots,L_h$$. Each list $$L_r$$ contains a subset of the items in $$L_{r-1}$$. We start with the input list $$L_0$$ that contains $$\mathtt{n}$$ items and construct $$L_1$$ from $$L_0$$, $$L_2$$ from $$L_1$$, and so on. The items in $$L_r$$ are obtained by tossing a coin for each element, $$\mathtt{x}$$, in $$L_{r-1}$$ and including $$\mathtt{x}$$ in $$L_r$$ if the coin turns up as heads. This process ends when we create a list $$L_r$$ that is empty. An example of a skiplist is shown in Figure $$\PageIndex{1}$$. Figure $$\PageIndex{1}$$: A skiplist containing seven elements.

For an element, $$\mathtt{x}$$, in a skiplist, we call the height of $$\mathtt{x}$$ the largest value $$r$$ such that $$\mathtt{x}$$ appears in $$L_r$$. Thus, for example, elements that only appear in $$L_0$$ have height 0. If we spend a few moments thinking about it, we notice that the height of $$\mathtt{x}$$ corresponds to the following experiment: Toss a coin repeatedly until it comes up as tails. How many times did it come up as heads? The answer, not surprisingly, is that the expected height of a node is 1. (We expect to toss the coin twice before getting tails, but we don't count the last toss.) The height of a skiplist is the height of its tallest node.

At the head of every list is a special node, called the sentinel, that acts as a dummy node for the list. The key property of skiplists is that there is a short path, called the search path, from the sentinel in $$L_h$$ to every node in $$L_0$$. Remembering how to construct a search path for a node, $$\mathtt{u}$$, is easy (see Figure $$\PageIndex{2}$$) : Start at the top left corner of your skiplist (the sentinel in $$L_h$$) and always go right unless that would overshoot $$\mathtt{u}$$, in which case you should take a step down into the list below.

More precisely, to construct the search path for the node $$\mathtt{u}$$ in $$L_0$$, we start at the sentinel, $$\mathtt{w}$$, in $$L_h$$. Next, we examine $$\texttt{w.next}$$. If $$\texttt{w.next}$$ contains an item that appears before $$\mathtt{u}$$ in $$L_0$$, then we set $$\mathtt{w}=\texttt{w.next}$$. Otherwise, we move down and continue the search at the occurrence of $$\mathtt{w}$$ in the list $$L_{h-1}$$. We continue this way until we reach the predecessor of $$\mathtt{u}$$ in $$L_0$$. Figure $$\PageIndex{2}$$: The search path for the node containing $$4$$ in a skiplist.

The following result, which we will prove in Section 4.4, shows that the search path is quite short:

Lemma $$\PageIndex{1}$$.

The expected length of the search path for any node, $$\mathtt{u}$$, in $$L_0$$ is at most $$2\log \mathtt{n} + O(1) = O(\log \mathtt{n})$$.

A space-efficient way to implement a skiplist is to define a Node, $$\mathtt{u}$$, as consisting of a data value, $$\mathtt{x}$$, and an array, $$\mathtt{next}$$, of pointers, where $$\mathtt{u.next[i]}$$ points to $$\mathtt{u}$$'s successor in the list $$L_{\mathtt{i}}$$. In this way, the data, $$\mathtt{x}$$, in a node is referenced only once, even though $$\mathtt{x}$$ may appear in several lists.

    class Node<T> {
T x;
Node<T>[] next;
Node(T ix, int h) {
x = ix;
next = (Node<T>[])Array.newInstance(Node.class, h+1);
}
int height() {
return next.length - 1;
}
}


The next two sections of this chapter discuss two different applications of skiplists. In each of these applications, $$L_0$$ stores the main structure (a list of elements or a sorted set of elements). The primary difference between these structures is in how a search path is navigated; in particular, they differ in how they decide if a search path should go down into $$L_{r-1}$$ or go right within $$L_r$$.

4.1: The Basic Structure is shared under a CC BY license and was authored, remixed, and/or curated by Pat Morin.