reverse_words.rb

# nil terminating strings are not used in ruby
# however for this question assume you are passed a nil terminated string
def string_reverse(string, from, to)
  if (!string || string.length < 2)
    return
  end
  while (from < to)
    string[from], string[to] = string[to], string[from]

    from +=1
    to -= 1
  end
  string
end

def reverse_words(sentence)
  # Here sentence is a nil-terminated string ending with char '\0'.
  if (!sentence || sentence.length == 0)
    return
  end

  #  To reverse all words in the string, we will first reverse
  #  the string. Now all the words are in the desired location, but
  #  in reverse order: "Hello World" -> "dlroW olleH".

  string_length = sentence.length
  sentence = string_reverse(sentence, 0, string_length - 1)

  # Now, s iterate the sentence and reverse each word in place.
  # "dlroW olleH" -> "World Hello"

  from = 0
  to = 0
  while (true)
    # find the  from index of a word while skipping spaces.
    while (sentence[from] == ' ')
      from += 1
    end
    if from >= sentence.length
      break
    end

    # find the to index of the word.
    to = from + 1
    while (to < sentence.length && sentence[to] != ' ')
      to += 1
    end

    # s reverse the word in-place.
    sentence = string_reverse(sentence, from, to - 1)

    from = to
  end
  sentence
end

p reverse_words('Hello World!')

# "World! Hello"