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+<h2><a href="https://leetcode.com/problems/fibonacci-number/">509. Fibonacci Number</a></h2><h3>Easy</h3><hr><div><p>The <b>Fibonacci numbers</b>, commonly denoted <code>F(n)</code> form a sequence, called the <b>Fibonacci sequence</b>, such that each number is the sum of the two preceding ones, starting from <code>0</code> and <code>1</code>. That is,</p>
+
+<pre>F(0) = 0, F(1) = 1
+F(n) = F(n - 1) + F(n - 2), for n &gt; 1.
+</pre>
+
+<p>Given <code>n</code>, calculate <code>F(n)</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre><strong>Input:</strong> n = 2
+<strong>Output:</strong> 1
+<strong>Explanation:</strong> F(2) = F(1) + F(0) = 1 + 0 = 1.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre><strong>Input:</strong> n = 3
+<strong>Output:</strong> 2
+<strong>Explanation:</strong> F(3) = F(2) + F(1) = 1 + 1 = 2.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre><strong>Input:</strong> n = 4
+<strong>Output:</strong> 3
+<strong>Explanation:</strong> F(4) = F(3) + F(2) = 2 + 1 = 3.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>0 &lt;= n &lt;= 30</code></li>
+</ul>
+</div>