check-completeness-of-a-binary-tree.md

<p>Given a binary tree, determine if it is a <em>complete binary tree</em>.</p>

<p><u><b>Definition of a complete binary tree from <a href="http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees" target="_blank">Wikipedia</a>:</b></u><br />
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2<sup>h</sup> nodes inclusive at the last level h.</p>

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<p><strong>Example 1:</strong></p>

<p><strong><img alt="" src="https://assets.leetcode.com/uploads/2018/12/15/complete-binary-tree-1.png" style="width: 180px; height: 145px;" /></strong></p>

<pre>
<strong>Input: </strong><span id="example-input-1-1">[1,2,3,4,5,6]</span>
<strong>Output: </strong><span id="example-output-1">true</span>
<span><strong>Explanation: </strong></span>Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.
</pre>

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<p><strong>Example 2:</strong></p>

<p><strong><img alt="" src="https://assets.leetcode.com/uploads/2018/12/15/complete-binary-tree-2.png" style="width: 200px; height: 145px;" /></strong></p>

<pre>
<strong>Input: </strong><span id="example-input-2-1">[1,2,3,4,5,null,7]</span>
<strong>Output: </strong><span id="example-output-2">false</span>
<strong>Explanation: </strong>The node with value 7 isn&#39;t as far left as possible.<span>
</span></pre>

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<p><strong>Note:</strong></p>

<ol>
	<li>The tree will have between 1 and 100 nodes.</li>
</ol>