Good implementation, but you should use std::isalpha instead of the 'a','z' check.

I like your approach, but I think it would be better to try to apply CPP standard libraries in your code. This Kata is easy and straight forward and people can understand it easily, while you sometimes have to write a long code or sctrip to do sth, then it might be problematic for other.

Oh, I like this solution. It changes the char 'c' to lower case and confirms its value is within 'a' to 'z'. Then it gets the numeric value of the char, subtracts 'a' from it (since each letter's numerical value sequentially follows), and then adds 1 to satisfy the requirement of the kata (since a-a=0 or z-a=25).