For the sake of completeness, I finally did manage...

# functional

[JP]

For the sake of completeness, I finally did manage to rewrite the recursive function into tail-recursive one using CPS.
Solution2 contains the recursive function, Solution3 contains the tail-recursive function without using CPS which I had to rethink the whole procedure, and Solution4 contains the tail-recursive function using CPS, converted from `Solution2`'s recursive one.

It was quite a brain twister for me, and I’m still not so confident if I’d be able to do this for other pieces of codes in general, but I think I did learn a bit.

One question arose:
Does the tail-recursive function converted with CPS from original recursive function always have the same space complexity, just that the space it occupies is not the stack space anymore? Or am I wrong about it?

If there are any comments, I’d be happy to hear.

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