The Typing Time Machine

You type word by word; undo unwrites the LAST word. Time travel for sentences! ⏪

You get tokens — words, or undo (never more undos than words). Print the surviving sentence with single spaces, or blank page.

Example:

Input: tokens = ["i", "love", "cats", "undo", "dogs"] Output: i love dogs

i, love, cats get pushed. undo pops cats off. Then dogs gets pushed — leaving i, love, dogs.