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leetcode/problem/student-attendance-record-ii.html

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+<p>An attendance record for a student can be represented as a string where each character signifies whether the student was absent, late, or present on that day. The record only contains the following three characters:</p>
+
+<ul>
+	<li><code>&#39;A&#39;</code>: Absent.</li>
+	<li><code>&#39;L&#39;</code>: Late.</li>
+	<li><code>&#39;P&#39;</code>: Present.</li>
+</ul>
+
+<p>Any student is eligible for an attendance award if they meet <strong>both</strong> of the following criteria:</p>
+
+<ul>
+	<li>The student was absent (<code>&#39;A&#39;</code>) for <strong>strictly</strong> fewer than 2 days <strong>total</strong>.</li>
+	<li>The student was <strong>never</strong> late (<code>&#39;L&#39;</code>) for 3 or more <strong>consecutive</strong> days.</li>
+</ul>
+
+<p>Given an integer <code>n</code>, return <em>the <strong>number</strong> of possible attendance records of length</em> <code>n</code><em> that make a student eligible for an attendance award. The answer may be very large, so return it <strong>modulo</strong> </em><code>10<sup>9</sup> + 7</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> n = 2
+<strong>Output:</strong> 8
+<strong>Explanation:</strong> There are 8 records with length 2 that are eligible for an award:
+&quot;PP&quot;, &quot;AP&quot;, &quot;PA&quot;, &quot;LP&quot;, &quot;PL&quot;, &quot;AL&quot;, &quot;LA&quot;, &quot;LL&quot;
+Only &quot;AA&quot; is not eligible because there are 2 absences (there need to be fewer than 2).
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> n = 1
+<strong>Output:</strong> 3
+</pre>
+
+<p><strong>Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> n = 10101
+<strong>Output:</strong> 183236316
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= n &lt;= 10<sup>5</sup></code></li>
+</ul>