c$++$ program to find the longest radical string.

Consider a string containing only letters of the English alphabet, which may contain

upper or lowercase characters. A string is considered radical if it contains both the

uppercase and lowercase equivalent of a letter.

For example $$XyYx$$ would be considered radical since it contains both an uppercase $$x$$

and a lowercase $$x$,$ and same for uppercase and lowercase $$y$.$

Your function longestRadical will take in a string and return the longest radical substring $($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 later occurring string $($relative to the original inputted string$).$