Yes, you're right, then i rewrite my solution

OP solved it, closing!

No, it is not "in an absolutely random time". It is always. For "abcb", your solution always return "cb" instead of "bc". Your solution has a bug, and handles some inputs incorrectly. You can also check "abc abc".

You need to check exactly, because for s="abcb", your solution does not return "bc" (as it should), but something else.

What will your solution return for string s="abcb"?

It is very difficult to tell why your code is wrong without seeing your code, or even without knowing what language you are trying to solve.