What Is a Skip List in Data Structure?

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Larry Thompson

A skip list is a data structure that provides an efficient way to search for elements in a collection. It combines the simplicity of a linked list with the speed of a binary search tree. In this article, we will explore what skip lists are, how they work, and why they are useful in certain scenarios.

What are Skip Lists?

Skip lists were first introduced by William Pugh in 1989 as an alternative to balanced binary search trees. They offer logarithmic search time complexity on average, similar to balanced trees, but with simpler implementation and better performance in some cases.

A skip list consists of layers, where each layer is essentially a linked list. The bottom layer contains all the elements in sorted order. Each higher layer skips some elements from the lower layer using special nodes called “skip” or “tower” nodes.

The skip nodes act as shortcuts between different levels of the linked list. By using these shortcuts, the search time can be significantly reduced compared to a regular linked list.

How do Skip Lists work?

Let’s say we have a skip list with four layers:

• Layer 4: A – C – D
• Layer 3: A – – D
• Layer 2: A – – D
• Layer 1: A – B – C – D

In this example, each element is represented by a letter (A, B, C, D). The bottom layer (layer 1) contains all the elements in sorted order.

To perform a search on the skip list for an element X:

1. Start at the top-left corner (the beginning of the top layer).
2. Move to the right until you find an element greater than or equal to X.
3. If the current element is equal to X, the search is successful.
4. Otherwise, move down one layer and repeat steps 2-3.
5. If you reach the bottom layer without finding X, the search is unsuccessful.

By moving down layers whenever possible, skip lists can eliminate a significant number of comparisons compared to a regular linked list. This makes them faster for searching elements.

Advantages of Skip Lists

Skip lists offer several advantages over other data structures:

• Efficient Search: Skip lists provide logarithmic search time complexity on average, making them suitable for large collections.
• Simplicity: Implementing skip lists is relatively straightforward compared to balanced trees like AVL or Red-Black trees.
• Flexibility: Skip lists can be easily modified and updated without affecting their overall structure or performance characteristics.

Conclusion

Skip lists are a powerful data structure that combines the simplicity of linked lists with efficient search capabilities. They provide a fast and scalable solution for searching elements in large collections.

By using skip nodes as shortcuts between different levels, skip lists can achieve logarithmic search time complexity on average. Understanding how skip lists work and their advantages can help you make informed decisions when designing and implementing data structures in your applications.