Build the PrimitiveMedium
Design Add and Search Words
Implement `WordDictionary` with `add_word(word)` and `search(word)`, where `.` in a search matches any single character.
add_word("bad"); search("b..") -> True
Without the wildcard this is a plain trie walk. With it, a `.` means recursing into *every* child and succeeding if any branch does. Returning on the first child that has a match at that depth is the usual bug.
What to expect: A timer starts when you begin. Edit the starter code, run it against the test suite as many times as you like, then finish when you're done. The reference solution and interviewer follow-up questions unlock only after you finish.