I know, you have probably solved this Kata. Moreover there are already answers in comments, but anyway...
2**n this is the for the sum of binominal coefs, but there is another formula for sum of squares.

Hey guys. I am totaly stuck... I`ve got an algorithm to calculate the ansver in O(1). All basic tests are passed. But I constantly fail all tests, where the lenght of string equals 23,24,25. Some scary stuff happens, always like this:

'noabcdefgijklymzustvpxwrq' should equal 'nmzyxwvutsrqpolkjigfedcba'
'lzyxvutsrqonmeijhkcafdgb' should equal 'lzyxvutsrqonmkjihgfedcba'

I mean that it goes in the right direction, but suddenly mistakes. Give me a hint what I should check or what thing to fix. Thanks a lot!