A substitution cipher is a simple method of encoding text in which all occurrences of each letter are replaced by another. For example, consider the following substitution table:
original: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
encoded: P X M S A C Z E V G I J D K N F O R L H T U W B Y Q
The message "REGISTER NOW FOR THE TOPCODER OPEN" would be encoded as "RAZVLHAR KNW CNR HEA HNFMNSAR NFAK" by finding each letter in the top row and replacing it with the letter directly beneath.
Substitution ciphers are relatively easy to break. One method takes advantage of the fact that some letters are used more frequently than others. For example, in English text, the letter 'E' occurs more often than any other. It is reasonable to guess that the most common letter in the encoded text represents 'E'.
You will be given the encoded text as a String encoded. You will also be given a String frequencyOrder containing all the letters that occur in the unencoded text sorted in decreasing order by frequency.
You are to attempt to decipher the encoded text by replacing the most frequent letter in encoded
with frequencyOrder, the second-most frequent letter in encoded with frequencyOrder, and so on. If some letters occur equally frequently in encoded, replace the letter that occurs first alphabetically with the lower-index character in frequencyOrder.
Your method should return a String with exactly the same number of elements, and characters in each element, as encoded. Replace only letters, preserving spaces.