Ya, it looks like you are removing the correct answer from the string itself. I recommend duplicating the input string before you begin to modify it, that way the input stays the same.

I'm not sure which language you use, apparently you mutate numbers:

Do not mutate the original array/list.

Ruby (and probably Crystal and other languages too): strings are mutable and random test is vulnerable to input modification.

Resolved for a better issue (raised above)

Did you mutate the input string?

I'm not quite sure why the random tests in ruby are expected to display values outside the middle of the string. Maybe I'm missing something. Can someone explain why expected values exclude the middle characters for random tests and not for primary tests?

Testing for bLXT
It should work for random inputs too - Expected: "bT", instead got: "LX"
Testing for hBRnHu
It should work for random inputs too - Expected: "BH", instead got: "Rn"
Testing for FSCfE
It should work for random inputs too - Expected: "Sf", instead got: "C"