A trie (pronounced as "try") or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker.

Implement the Trie class:

- `Trie()` Initializes the trie object.
- `void insert(String word)` Inserts the string `word` into the trie.
- `boolean search(String word)` Returns `true` if the string `word` is in the trie (i.e., was inserted before), and `false` otherwise.
- `boolean startsWith(String prefix)` Returns `true` if there is a previously inserted string `word` that has the prefix `prefix`, and `false` otherwise.

Example 1:

    Input
    ["Trie", "insert", "search", "search", "startsWith", "insert", "search"]
    [[], ["apple"], ["apple"], ["app"], ["app"], ["app"], ["app"]]
    Output
    [null, null, true, false, true, null, true]

    Explanation
    Trie trie = new Trie();
    trie.insert("apple");
    trie.search("apple");   // return True
    trie.search("app");     // return False
    trie.startsWith("app"); // return True
    trie.insert("app");
    trie.search("app");     // return True

Constraints:

- `1 <= word.length, prefix.length <= 2000`
- `word` and `prefix` consist only of lowercase English letters.
- At most `3 * 104` calls in total will be made to `insert`, `search`, and `startsWith`.