count_letters.rb

def count_letters(letter)
  hash = Hash.new { |hash,key| hash[key] = [] }

  letter.split("").each_with_index() { |x,i|
     hash[x] << i
  }
  
  hash
end

puts count_letters("lighthouse in the house...")

Output is:

{"l"=>[0], "i"=>[1, 11], "g"=>[2], "h"=>[3, 5, 15, 18], "t"=>[4, 14], "o"=>[6, 19], "u"=>[7, 20], "s"=>[8, 21], "e"=>[9, 16, 22], " "=>[10, 13, 17], "n"=>[12], "."=>[23, 24, 25]}