Skip Lists
In this section, we consider an approach to developing fast implementations of symbol-table operations which seems at first to be completely different from the tree-based methods that we have been considering; but it actually is closely related to them. The approach is based on a randomized data structure and is almost certain to provide near-optimal performance for all the basic operations for the symbol-table ADT that we have been considering. The underlying data structure, which was developed by Pugh in 1990 (see reference section), is called a skip list. It uses extra links in the nodes of a linked list to skip through large portions of a list at a time during a search.
Screenshot gives a simple example, where every third node in an ordered linked list contains an extra link that allows us to skip three nodes in the list. We can use the extra links to speed up search: We scan through the top list until we find the key or a node with a smaller key with a link to a node with a larger key, then use the links at the bottom to check the two intervening nodes. This method speeds up search by a factor of 3, because we examine only about k/3 nodes in a successful search for the kth node on the list.
Screenshot A two-level linked list
Every third node in this list has a second link, so we can skip through the list at nearly three times the speed that we could go by following the first links. For example, we can get to the twelfth node in the list, the P, from the beginning by following just five links: second links to C, G, L, N, and then through N's first link, P.
We can iterate this construction and provide a second extra link to be able to scan faster through the nodes with extra links, and so forth. Also, we can generalize the construction by skipping a variable number of nodes with each link.
Definition 13.5 A skip list is an ordered linked list where each node contains a variable number of links, with the ith links in the nodes implementing singly linked lists that skip the nodes with fewer than i links.
Screenshot depicts a sample skip list and shows an example of searching and inserting a new node. To search, we scan through the top list until we find the search key or a node with a smaller key that has a link to a node with a larger key; then, we move to the second-from-top list and iterate the procedure, continuing until the search key is found or a search miss happens on the bottom level. To insert, we search, linking in the new node when moving from level k to level k - 1 if the new node has at least k extra links.
Screenshot Search and insertion in a skip list
By adding more levels to the structure in Screenshot and allowing links to skip variable numbers of nodes, we get an example ofa general skip list. To search for a key in the list, we start at the highest level, moving down each time that we encounter a key that is not smaller than the search key. Here (top), we find L by starting at level 3, moving across the first link, then down at G (treating the null link as a link to a sentinel), then across to I, then down to level 2 because S is greater than L, then down to level 1 because M is greater than L. To insert a node L with three links, we link it into the three lists at precisely the places where we found links to greater keys during the search.
The internal representation of the nodes is straightforward. We replace the single link in a singly linked list by an array of links, and an integer that contains the number of links in the node. Memory management is perhaps the most complicated aspect of skip lists- we will examine the type declarations and the code for allocating new nodes shortly, when we consider insertion. For the moment, it suffices to note that we can access the node that follows node t on the (k + 1)st level in the skip list by accessing t.next [k]. The recursive implementation in Program 13.7 shows that searching in skip lists not only is a straightforward generalization of searching in singly linked lists, but also is similar to binary search or searching in BSTs. We test whether the current node has the search key; if it does not, we compare the key in the current node with the search key. We do one recursive call if it is larger and a different recursive call if it is smaller.
Searching in skip lists
For k equal to 0, this code is equivalent to Program 12.6, for searching in singly linked lists. For general k, we move to the next node in the list on level k if its key is smaller than the search key, and down to level k-1 if its key is not smaller.
private ITEM searchR(Node t, KEY v, int k) { if (t == null) return null; if (t != head) if (equals(t.item.key(), v)) return t.item; if (k >= t.sz) k = t.sz-1; if (t.next[k] != null) if (!less(v, t.next[k].item.key())) return searchR(t.next[k], v, k); return (k == 0) ? null : searchR(t, v, k-1); } ITEM search(KEY v) { return searchR(head, v, lgN - 1); }
The first task that we face when we want to insert a new node into a skip list is to determine how many links we want that node to have. All the nodes have at least one link; following the intuition depicted in Screenshot, we can skip t nodes at a time on the second level if one out of every t nodes has at least two links; iterating, we come to the conclusion that we want one out of every tj nodes to have at least j + 1 links.
Skip-list data structures and constructor
Nodes in skip lists have an array of links, so the constructor for node needs to allocate the array and to set all the links to null. The constant L is the maximum number of levels that we will allow in the list: it might be set to the logarithm of the expected number of items on the list. The variable N keeps the number of items in the list, as usual, and lgN is the number of levels. An empty list is a head node with L links, all set to 0, with N and lgN also set to 0.
private class Node { ITEM item; Node[] next; int sz; Node(ITEM x, int k) { item = x; sz = k; next = new Node[sz]; } } private static final int L = 50; private Node head; private int N, lgN; ST(int maxN) { N = 0; lgN = 0; head = new Node(null, L); }
To make nodes with this property, we randomize, using a method that returns j + 1 with probability 1/tj. Given j, we create a new node with j links and insert it into the skip list using the same recursive schema as we did for search, as illustrated in Screenshot. After we have reached level j, we link in the new node each time that we move down to the next level. At that point, we have established that the item in the current node is less than the search key and links (on level j) to a node that is not less than the search key.
To initialize a skip list, we build a head node with the maximum number of levels that we will allow in the list, with null links at all levels. Programs 13.8 and 13.9 implement initialization and insertion for skip lists.
Screenshot shows the construction of a skip list for a sample set of keys when inserted in random order; Screenshot shows the construction of a skip list for the same set of keys as in Screenshot, but inserted in increasing order; and Screenshot shows a larger example. Like those of randomized BSTs, the stochastic properties of skip lists do not depend on the order in which keys are inserted.
Screenshot Skip-list construction
This sequence depicts the result of inserting items with keys A S E R C H I N G into an initially empty skip list. Nodes have (j + 1) links with probability 1/2j.
Screenshot Skip-list construction with keys in order
This sequence depicts the result of inserting items with keys A C E G H I N R S into an initially empty skip list. Stochastic properties of the list do not depend on the key insertion order.
Screenshot A large skip list
This skip list is the result of inserting 50 randomly ordered keys into an initially empty list. We can access any node by following 8 or fewer links.
Insertion in skip lists
We generate a new j-link node with probability 1/2j, then follow the search path precisely as in Program 13.7, but link in the new node when we move down to each of the bottom j levels.
private int randX() { int i, j; double t = Math.random(); for (i = 1, j = 2; i < L; i++, j += j) if (t*j > 1.0) break; if (i > lgN) lgN = i; return i; } private void insertR(Node t, Node x, int k) { KEY v = x.item.key(); Node tk = t.next[k]; if ((tk == null) || less(v, tk.item.key())) { if (k < x.sz) { x.next[k] = tk; t.next[k] = x; } if (k == 0) return; insertR(t, x, k-1); return; } insertR(tk, x, k); } void insert(ITEM v) { insertR(head, new Node(v, randX()), lgN); N++; }
Property 13.10
Search and insertion in a randomized skip list with parameter t require about (t logt N)/2 = (t/(2 lg t)) lg N comparisons, on the average.
We expect the skip list to have about logt N levels, because logt N is greater than the smallest j for which tj = N. On each level, we expect that about t nodes were skipped on the previous level and that we should have to go through about half of them, on the average, before dropping to the next level. The number of levels is small, as is clear from the example in Screenshot, but the precise analysis that establishes this is not elementary (see reference section). 
Property 13.11
Skip lists have (t/(t - 1))N links on the average.
There are N links on the bottom, N/t links on the first level, about N/t2 links on the second level, and so forth, for a total of about
links in the whole list.
Picking an appropriate value of t leads us immediately to a time- space tradeoff. When t = 2, skip lists need about lg N comparisons and 2N links, on the average-performance comparable with the best that we have seen with BSTs. For larger t, the time for search and insert is longer, but the extra space for links is smaller. Differentiating the expression in Property 13.10, we find that the choice t = e minimizes the expected number of comparisons for searching in a skip list. The following table gives the value of the coefficient of N lg N in the number of comparisons needed to construct a table of N items:
| t | 2 | e | 3 | 4 | 8 | 16 |
| lg t | 1.00 | 1.44 | 1.58 | 2.00 | 3.00 | 4.00 |
| t/lg t | 2.00 | 1.88 | 1.89 | 2.00 | 2.67 | 4.00 |
If doing comparisons, following links, and moving down recursively have costs that differ substantially, we can do a more refined calculation along these lines (see Exercise 13.83).
Because the search time is logarithmic, we can reduce the space overhead to not much more than that for singly linked lists (if space is tight) by increasing t. Precise estimates of running time depend on assessment of the relative costs of following links across the lists and the recursive calls to move down to the next level. We shall revisit this kind of time-space tradeoff again in , when we look at the problem of indexing huge files.
Other symbol-table methods are straightforward to implement with skip lists. For example, Program 13.10 gives an implementation of the remove operation, using the same recursive scheme that we used for insert in Program 13.9. To delete, we unlink the node from the lists at each level (where we linked it in for insert), and we free the node after unlinking it from the bottom list (as opposed to creating it before traversing the list for insert). To implement join, we merge the lists (see Exercise 13.78); to implement select, we add a field to each node that gives the number of nodes skipped by the highest-level link to it (see Exercise 13.77).
Removal in skip lists
To remove a node with a given key from a skip list, we unlink it at each level that we find a link to it, then delete it when we reach the bottom level.
private void removeR(Node t, KEY v, int k) { Node x = t.next[k]; if (!less(x.item.key(), v)) { if (equals(v, x.item.key())) { t.next[k] = x.next[k]; } if (k == 0) return; removeR(t, v, k-1); return; } removeR(t.next[k], v, k); } void remove(ITEM x) { removeR(head, x.key(), lgN); N--; }
Although skip lists are easy to conceptualize as a systematic way to move quickly through a linked list, it is also important to understand that the underlying data structure is nothing more than an alternative representation of a balanced tree. For example, Screenshot shows the skip-list representation of the balanced 2-3-4 tree in Screenshot. We can implement the balanced 2-3-4 tree algorithms of using the skip-list abstraction, rather than the red-black tree abstraction of . The resulting code is somewhat more complicated than the implementations that we have considered (see Exercise 13.80). We shall revisit this relationship between skip lists and balanced trees in .
Screenshot Skip-list representation of a 2-3-4 tree
This skip list is a representation of the 2-3-4 tree in Screenshot. In general, skip lists correspond to balanced multiway trees with one or more links per node (1-nodes, with no keys and 1 link, are allowed). To build the skip list corresponding to a tree, we give each node a number of links equal to its height in the tree, and then link the nodes horizontally. To build the tree corresponding to a skip list, we group skipped nodes and recursively link them to nodes at the next level.
The ideal skip list illustrated in Screenshot is a rigid structure that is as difficult to maintain, when we insert a new node, as is the ordered array for binary search, because the insertion involves changing all the links in all the nodes after the node inserted. One way to loosen the structure is to build lists where each link skips either one, two, or three links on the level below: this arrangement corresponds to 2-3-4 trees, as illustrated in Screenshot. The randomized algorithm discussed in this section is another effective way to loosen the structure; we shall consider other alternatives in .
Exercises
Draw the skip list that results when you insert items with the keys E A S Y Q U T I O N in that order into an initially empty list, assuming that randX returns the sequence of values 1, 3, 1, 1, 2, 2, 1, 4, 1, and 1.
13.76 Draw the skip list that results when you insert items with the keys A E I N O Q S T U Y in that order into an initially empty list, assuming the same randX return values as for Exercise 13.75.
Implement the select operation for a skip-list-based symbol table.
13.78 Implement the join operation for a skip-list-based symbol table.
Use skip lists to implement construct, search, and insert for symbol tables with the balanced 2-3-4 tree abstraction.
Choose the value of t that minimizes the search cost for the case that following a link costs a times as much as doing a comparison and that moving down one level of recursion costs b times as much as doing a comparison.