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

pushing a string onto a stack and then popping it off - is that still O(1)? essentially, that is the same as using split()...

Doesn't O(1) mean in-place? This task gets easy if we can just append strings and stuff, but that uses space...

EDIT: Thomas Watnedal is right. The following algorithm is O(n) in time and O(1) in space:

1. reverse string in-place (first iteration over string)
2. reverse each (reversed) word in-place (another two iterations over string)
   1. find first word boundary
   2. reverse inside this word boundary
   3. repeat for next word until finished

I guess we would need to prove that step 2 is really only O(2n)...