Count the Number of Special Characters I
<p>You are given a string <code>word</code>. A letter is called <strong>special</strong> if it appears <strong>both</strong> in lowercase and uppercase in <code>word</code>.</p>

<p>Return the number of<em> </em><strong>special</strong> letters in<em> </em><code>word</code>.</p>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">word = &quot;aaAbcBC&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">3</span></p>

<p><strong>Explanation:</strong></p>

<p>The special characters in <code>word</code> are <code>&#39;a&#39;</code>, <code>&#39;b&#39;</code>, and <code>&#39;c&#39;</code>.</p>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">word = &quot;abc&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">0</span></p>

<p><strong>Explanation:</strong></p>

<p>No character in <code>word</code> appears in uppercase.</p>
</div>

<p><strong class="example">Example 3:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">word = &quot;abBCab&quot;</span></p>

<p><strong>Output:</strong> 1</p>

<p><strong>Explanation:</strong></p>

<p>The only special character in <code>word</code> is <code>&#39;b&#39;</code>.</p>
</div>

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

<ul>
	<li><code>1 &lt;= word.length &lt;= 50</code></li>
	<li><code>word</code> consists of only lowercase and uppercase English letters.</li>
</ul>
Hash Table; String
function numberOfSpecialChars(word: string): number {
    const s: boolean[] = Array.from({ length: 'z'.charCodeAt(0) + 1 }, () => false);
    for (let i = 0; i < word.length; ++i) {
        s[word.charCodeAt(i)] = true;
    }
    let ans: number = 0;
    for (let i = 0; i < 26; ++i) {
        if (s['a'.charCodeAt(0) + i] && s['A'.charCodeAt(0) + i]) {
            ++ans;
        }
    }
    return ans;
}
