# Skip Lists

Skip lists are linked lists that allow you to skip to the correct node. The performance bottleneck inherent in a sequential scan is avoided, while insertion and deletion remain relatively efficient. Average search time is O(lg n). Worst-case search time is O(n), but is extremely unlikely. An excellent reference for skip lists is Pugh [1990].

## Theory

The indexing scheme employed in skip lists is similar in nature to the method used to lookup names in an address book. To lookup a name, you index to the tab representing the first character of the desired entry. In Figure 3-8, for example, the top-most list represents a simple linked list with no tabs. Adding tabs (middle figure) facilitates the search. In this case, level-1 pointers are traversed. Once the correct segment of the list is found, level-0 pointers are traversed to find the specific entry.

### Figure 3-8: Skip List Construction

The indexing scheme may be extended as shown in the bottom figure, where we now have an index to the index. To locate an item, level-2 pointers are traversed until the correct segment of the list is identified. Subsequently, level-1 and level-0 pointers are traversed.

During insertion the number of pointers required for a new node must be determined. This is easily resolved using a probabilistic technique. A random number generator is used to toss a computer coin. When inserting a new node, the coin is tossed to determine if it should be level-1. If you win, the coin is tossed again to determine if the node should be level-2. Another win, and the coin is tossed to determine if the node should be level-3. This process repeats until you lose. If only one level (level-0) is implemented, the data structure is a simple linked-list with O(n) search time. However, if sufficient levels are implemented, the skip list may be viewed as a tree with the root at the highest level, and search time is O(lg n).

The skip list algorithm has a probabilistic component, and thus a probabilistic bounds on the time required to execute. However, these bounds are quite tight in normal circumstances. For example, to search a list containing 1000 items, the probability that search time will be 5 times the average is about 1 in 1,000,000,000,000,000,000.

## Implementation

An ANSI-C implementation for skip lists is included. Typedef T and comparison operators compLT and compEQ should be altered to reflect the data stored in the list. In addition, MAXLEVEL should be set based on the maximum size of the dataset.

To initialize, initList is called. The list header is allocated and initialized. To indicate an empty list, all levels are set to point to the header. Function insertNode allocates a new node, searches for the correct insertion point, and inserts it in the list. While searching, the update array maintains pointers to the upper-level nodes encountered. This information is subsequently used to establish correct links for the newly inserted node. The newLevel is determined using a random number generator, and the node allocated. The forward links are then established using information from the update array. Function deleteNode deletes and frees a node, and is implemented in a similar manner. Function findNode searches the list for a particular value.