Checks whether a string is a palindrome using recursion.

palindrome.rb

def palindrome str
  str.split('')
  if str.length <= 1
    return true
  elsif str.length == 2
    return str[0] == str[1]
  else
    length = str.length
    if str[0] == str[length-1]
      return palindrome str[1..length-2]
    else
      return false
    end
  end
end

puts palindrome ""         # => true
puts palindrome "racecar"  # => true
puts palindrome "parking"  # => false
puts palindrome "aa"       # => true
puts palindrome "ab"       # => false
puts palindrome "a"        # => true
puts palindrome "aba"      # => true
puts palindrome "abc"      # => false
puts palindrome "1991"     # => true

right_way.rb

# Or just do it the right way...

def palindrome(str)
  str == str.reverse
end

puts palindrome ""         # => true
puts palindrome "racecar"  # => true
puts palindrome "parking"  # => false
puts palindrome "aa"       # => true
puts palindrome "ab"       # => false
puts palindrome "a"        # => true
puts palindrome "aba"      # => true
puts palindrome "abc"      # => false
puts palindrome "1991"     # => true