(MEDIUM) Given an input String and a 26-character key String, encode the input by substituting each letter of the input with the character having the same index ('A'==0, 'B'==1, ..., 'Z'==25) from the key. Precondition: no lowercase letters in the plaintext, but non-letter characters are allowed and will not be substituted in the output. Note that the keys will have non-letter characters as well however, meaning that the output will have an increased number of non-letter characters as a result.