ruby.rb

# the last letter position
length = input.length - 1

# the first letter
first = input[0]

# the lat letter
last = input[length]

# replace the first letter with the last
input[0] = last

# replace the last letter with the first
input[length] = first


OR ---------- use regex

input.gsub!(/(^\S)(.*)(\S$)/,'\3\2\1')

(^\S) - first group - first letter
(.*) - second group - all letters
(\S$) - third group - last letter

\3\2\1 - show third group, second group, first group