"Take 2 strings s1 and s2 including only letters from a to z."

"Take 2 strings s1 and s2 including only letters from a to z." It's defined in the instructions, so the tests are complete.

"but still this solution seems to be correct, because task guarantees that only letters will be passed in. solution does not have to check for non-letters."

That is fair I suppose.

• solution has to be correct to be posted here.
• any solution is just as correct as far the tests go. If tests are poor, invalid solutions can slip through.
• but still this solution seems to be correct, because task guarantees that only letters will be passed in. solution does not have to check for non-letters.

I thought a solution had to be correct to be posted here?

This is incorrect. It does not test for non-leters.

...but why?

Thank you for your suggestion. I will add that information to the description.

It might be helpful to specify what to do about negative numbers, or include a test case for them.

Specifically, since the description said "integers" and not "positive integers" or "natural numbers", I scanned for /-?\d+/. This caused my code to fail on a randomly generated test case that included a dash before a decimal digit.

Oops. Bloated, comment packed trash.