[kaymanb]

Disjoint-Sets have a very cool implementation whose amortized time complexity is extremely slow growing. It is not quite constant, but even for a disjoint-set with as many elements as there are particles in the universe, the amortized cost of an operation will be less than or equal to 4.

https://en.wikipedia.org/wiki/Disjoint-set_data_structure

[andreareina]

Use the idea[1] at work to make large NP-complete problems more tractable by breaking the problem up into disjoint subsets.

[1] doesn't implement the inverse-ackermann algorithm but still implemented as union/find.

[kragen]

aka "union find" — some other comments in the thread call it that

[EnderShadow8]

I've been told to consider it constant for all practical purposes

[pjscott]

The inverse Ackermann function is one of the slowest-growing functions that you’re ever likely to encounter in the wild. To say that it’s “constant for all practical purposes” is 100% true but doesn’t do justice to how amazingly slow this function is to grow.

https://en.m.wikipedia.org/wiki/Ackermann_function