Implement Trie (Prefix Tree)
<p>A <a href="https://en.wikipedia.org/wiki/Trie" target="_blank"><strong>trie</strong></a> (pronounced as &quot;try&quot;) or <strong>prefix tree</strong> 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.</p>

<p>Implement the Trie class:</p>

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

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input</strong>
[&quot;Trie&quot;, &quot;insert&quot;, &quot;search&quot;, &quot;search&quot;, &quot;startsWith&quot;, &quot;insert&quot;, &quot;search&quot;]
[[], [&quot;apple&quot;], [&quot;apple&quot;], [&quot;app&quot;], [&quot;app&quot;], [&quot;app&quot;], [&quot;app&quot;]]
<strong>Output</strong>
[null, null, true, false, true, null, true]

<strong>Explanation</strong>
Trie trie = new Trie();
trie.insert(&quot;apple&quot;);
trie.search(&quot;apple&quot;);   // return True
trie.search(&quot;app&quot;);     // return False
trie.startsWith(&quot;app&quot;); // return True
trie.insert(&quot;app&quot;);
trie.search(&quot;app&quot;);     // return True
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= word.length, prefix.length &lt;= 2000</code></li>
	<li><code>word</code> and <code>prefix</code> consist only of lowercase English letters.</li>
	<li>At most <code>3 * 10<sup>4</sup></code> calls <strong>in total</strong> will be made to <code>insert</code>, <code>search</code>, and <code>startsWith</code>.</li>
</ul>
Design; Trie; Hash Table; String
class TrieNode {
    children;
    isEnd;
    constructor() {
        this.children = new Array(26);
        this.isEnd = false;
    }
}

class Trie {
    root;
    constructor() {
        this.root = new TrieNode();
    }

    insert(word: string): void {
        let head = this.root;
        for (let char of word) {
            let index = char.charCodeAt(0) - 97;
            if (!head.children[index]) {
                head.children[index] = new TrieNode();
            }
            head = head.children[index];
        }
        head.isEnd = true;
    }

    search(word: string): boolean {
        let head = this.searchPrefix(word);
        return head != null && head.isEnd;
    }

    startsWith(prefix: string): boolean {
        return this.searchPrefix(prefix) != null;
    }

    private searchPrefix(prefix: string) {
        let head = this.root;
        for (let char of prefix) {
            let index = char.charCodeAt(0) - 97;
            if (!head.children[index]) return null;
            head = head.children[index];
        }
        return head;
    }
}
