Introduction to Fast and Slow Pointers

Let’s go over the Fast and Slow Pointers pattern, its real-world applications, and some problems we can solve with it.

About the pattern

Similar to the two pointers pattern, the fast and slow pointers pattern uses two pointers to traverse an iterable data structure, but at different speeds, often to identify patterns, detect cycles, or find specific elements. The speeds of the two pointers can be adjusted according to the problem statement. Unlike the two pointers approach, which is concerned with data values, the fast and slow pointers approach is often used to determine specific pattern or structure in the data.

The key idea is that the pointers start at the same location and then start moving at different speeds. Generally, the slow pointer moves forward by a factor of one, and the fast pointer moves by a factor of two. This approach enables the algorithm to detect patterns or properties within the data structure, such as cycles or intersections. If there is a cycle, the two are bound to meet at some point during the traversal. To understand the concept, think of two runners on a track. While they start from the same point, they have different running speeds. If the track is circular, the faster runner will overtake the slower one after completing a lap.

Here’s a simple demonstration of how the fast and slow pointers move along a data structure:

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