Added README.md file for Diameter of Binary Tree

Author: audemed44

543-diameter-of-binary-tree/README.md

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+<h2><a href="https://leetcode.com/problems/diameter-of-binary-tree">Diameter of Binary Tree</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Given the <code>root</code> of a binary tree, return <em>the length of the <strong>diameter</strong> of the tree</em>.</p>
+
+<p>The <strong>diameter</strong> of a binary tree is the <strong>length</strong> of the longest path between any two nodes in a tree. This path may or may not pass through the <code>root</code>.</p>
+
+<p>The <strong>length</strong> of a path between two nodes is represented by the number of edges between them.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/03/06/diamtree.jpg" style="width: 292px; height: 302px;" />
+<pre>
+<strong>Input:</strong> root = [1,2,3,4,5]
+<strong>Output:</strong> 3
+<strong>Explanation:</strong> 3 is the length of the path [4,2,1,3] or [5,2,1,3].
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [1,2]
+<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>[1, 10<sup>4</sup>]</code>.</li>
+	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
+</ul>