Thank you for the explanation , it is clear, just i need please a clarification about when we inverted the binary 1000 how we obtain -1001 ?

Hello , I find your solution as more efficient way. I have juste question , can you explain to me the following line of our code please?

c = static_cast('A' + (c + n - 'A') % 26 + (isEven ? 0 : 32));

thank you in advance

I have passed all tests

No you didn't, the timeout happened before you could complete all of them, your code isn't fast enough to pass them all, try another approach, O(n²) solutions won't work.