leetcode-problemset/leetcode/problem/count-the-number-of-special-characters-ii.html

<p>You are given a string <code>word</code>. A letter&nbsp;<code>c</code> is called <strong>special</strong> if it appears <strong>both</strong> in lowercase and uppercase in <code>word</code>, and <strong>every</strong> lowercase occurrence of <code>c</code> appears before the <strong>first</strong> uppercase occurrence of <code>c</code>.</p>

<p>Return the number of<em> </em><strong>special</strong> letters<em> </em>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 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>There are no special characters in <code>word</code>.</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> <span class="example-io">0</span></p>

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

<p>There are no special characters in <code>word</code>.</p>
</div>

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

<ul>
	<li><code>1 &lt;= word.length &lt;= 2 * 10<sup>5</sup></code></li>
	<li><code>word</code> consists of only lowercase and uppercase English letters.</li>
</ul>