In C++ please.

Consider a string containing only letters of the English alphabet, which may contain upper or lowercase characters. A string is considered awesome if it contains both the uppercase and lowercase equivalent of a letter.

For example XyYx would be considered awesome since it contains both an uppercase x and a lowercase x, and same for uppercase and lowercase y.

Given a string the user enters (assume the user is benevolent and cannot produce an output that will error your code, that is to say worry about only [A...Za...z]) find the longest occurring substring using a divide and conquer approach. If multiple strings would be tied for the longest overall substring length return the earliest occurring string (relative to the original inputted string).