count-complete-tree-nodes

Count Complete Tree Nodes

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countNodes

Given the <code>root</code> of a complete binary tree, return the number of the nodes in the tree.

According to <a href="http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees" target="_blank">Wikipedia</a>, 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>2h</code> nodes inclusive at the last level <code>h</code>.

Design an algorithm that runs in less than&nbsp;<code data-stringify-type="code">O(n)</code>&nbsp;time complexity.

&nbsp;
Example 1:
<img alt="" src="https://assets.leetcode.com/uploads/2021/01/14/complete.jpg" style="width: 372px; height: 302px;" />
<pre>
Input: root = [1,2,3,4,5,6]
Output: 6
</pre>

Example 2:

<pre>
Input: root = []
Output: 0
</pre>

Example 3:

<pre>
Input: root = [1]
Output: 1
</pre>

&nbsp;
Constraints:

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 5 * 104]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 5 * 104</code></li>
	<li>The tree is guaranteed to be complete.</li>
</ul>

Binary Search

Tree

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