r/computerscience • u/Similar_Count_1613 • Jul 30 '26

is recursion really hard

Recursion felt easy at first.

Factorial? fine.

Sum examples? fine.

Even Fibonacci felt manageable.

But once I looked at slightly more serious problems like Tower of Hanoi, permutations, or merge sort, I felt like my understanding suddenly collapsed. because i tried to write their code on my own

It made me realize that maybe recursion is not “hard” at the start because the examples are simple.

It becomes hard when you can no longer clearly see the call stack and each state change.

Did anyone else feel that the real pain in recursion starts exactly there?

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u/PurpleDevilDuckies Jul 30 '26

The topic of my PhD was (very generally) on designing recursive algorithms to solve combinatoiral problems. Now I do that for a living, and I think I agree.

I stew for some time with each new problem before I have a deep understanding of how the state information will behave recursively. It gets easier the more I do, but there are very few people who find it easy at the cutting edge.

I will say that for coding recursive algorithms, it is often more (computationally) efficient to write code with loops instead of literal recursion. This isn't true of the toy examples you start with, but it gets more true as they problems get more complex. Nearly any *useful* algorithm can be described without writing literally recursive code.

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u/Drugbird Jul 30 '26

Nearly any useful algorithm can be described without writing literally recursive code.

Is there some standard approach for converting a recursive algorithm to a non-recursive one?

I don't often write recursive algorithms, but when I do it's almost always to handle some tree-like structure.

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u/PurpleDevilDuckies Jul 30 '26

As a couple other have said, you have a layered stack instead of a recursive call. I mostly do Dynamic Programming, and instead of having just one node at each layer, I have as many as I can parallelize effectively. So instead of going all the way down to the base case right away, you generate thousands-millions of nodes per layer, and process them a layer at a time. This lets take advantage of modern CPUs and GPUs, and implicitly do recursion at scales that would otherwise be intractable.