Create a one-dimensional array to read 20 alphabetical letters (your program should be able to detect and print out an error message if a non-alphabetical letter is entered). As each letter is entered, print a message saying duplicate letter if the letter is already in the array. Write a function that can sort the array after all 20 letters have been entered. Write another function that print out the most frequent letter and number of times it was entered. Prepare for the case where all 20 letters are different, or all are the same