Efficiently reverse the order of the words (not characters) in an array of characters

Question

Given an array of characters which forms a sentence of words, give an efficient algorithm to reverse the order of the words (not characters) in it.

Example input and

Answer 1

O(N) in space and O(N) in time solution in Python:

def reverse_words_nosplit(str_):
  """
  >>> f = reverse_words_nosplit
  >>> f("this is a string")
  'string a is this'
  """
  iend = len(str_)
  s = ""
  while True:
    ispace = str_.rfind(" ", 0, iend)
    if ispace == -1:
      s += str_[:iend]
      break
    s += str_[ispace+1:iend]
    s += " "
    iend = ispace
  return s