What does "in-place" mean?
semantics
Solution
In-place means that you should update the original string rather than creating a new one.
Depending on the language/framework that you're using this could be impossible. (For example, strings are immutable in .NET and Java, so it would be impossible to perform an in-place update of a string without resorting to some evil hacks.)
Problem
Reverse words in a string (words are separated by one or more spaces). Now do it in-place. What does in-place mean?