0 <= input.length <= 100

Strings of lengths 0 and 100 should be tested.

input may contain any valid ASCII characters

This should be tested too.
Random tests should be added.
The function name should be valid-parentheses or maybe even valid-parentheses?.

I just wanted to add a simple fork that shows off what the Project Euler question was hinting towards. The iterative solution becomes slow beyond a million or so.

Thanks :).

I like the zipWith tail trick, I'll have to remember that for code golf.

Good catch, fixed.

Needs clojure examples added to description.

head a :: Int. Not a :: Int.

Cool, thanks. Works for me now!

Added.

Fixed again, made a slight typo.

Still seems to be doing it in Ruby:

Test number 6:
Test Passed: Value == 33
Test Passed: Value == []
Expected: [33], instead got: []
Test Passed: Value == 33

So the test expects init([33]) == [33]. Is that correct?

Yeah, I just noticed that my changes weren't saved, and I've updated them again.

No, the issue still occurs for JS:

// input: [ 12, 31, 38, 24 ]
Expected: [12,31], instead got: [12,31,38]

Rectified.

No, I do specify differently; the test cases were wrong, as indicated by the issues below.