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leetcode/problem/count-complete-tree-nodes.html

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+<p>Given the <code>root</code> of a <strong>complete</strong> binary tree, return the number of the nodes in the tree.</p>
+
+<p>According to <strong><a href="http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees" target="_blank">Wikipedia</a></strong>, every level, except possibly the last, is completely filled in a complete binary tree, and all nodes in the last level are as far left as possible. It can have between <code>1</code> and <code>2<sup>h</sup></code> nodes inclusive at the last level <code>h</code>.</p>
+
+<p>Design an algorithm that runs in less than&nbsp;<code data-stringify-type="code">O(n)</code>&nbsp;time complexity.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/01/14/complete.jpg" style="width: 372px; height: 302px;" />
+<pre>
+<strong>Input:</strong> root = [1,2,3,4,5,6]
+<strong>Output:</strong> 6
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = []
+<strong>Output:</strong> 0
+</pre>
+
+<p><strong>Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [1]
+<strong>Output:</strong> 1
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[0, 5 * 10<sup>4</sup>]</code>.</li>
+	<li><code>0 &lt;= Node.val &lt;= 5 * 10<sup>4</sup></code></li>
+	<li>The tree is guaranteed to be <strong>complete</strong>.</li>
+</ul>