Implement a Trie (prefix tree) with these operations:
Trie(): Initialize the trie.
insert(word): Insert a word into the trie.
search(word): Return true if the exact word is in the trie.
startsWith(prefix): Return true if any word in the trie starts with the given prefix.
With operations: insert("apple"), search("apple") returns true, search("app") returns false, startsWith("app") returns true, insert("app"), search("app") returns true.
Constraints: Words and prefixes contain only lowercase English letters.