Create readme : 0101-symmetric-tree

Author: SHIMHUN

LeetCode/Easy/0101-symmetric-tree/README.md

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+<h2><a href="https://leetcode.com/problems/symmetric-tree">101. Symmetric Tree</a></h2><h3>Easy</h3><hr><p>Given the <code>root</code> of a binary tree, <em>check whether it is a mirror of itself</em> (i.e., symmetric around its center).</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/symtree1.jpg" style="width: 354px; height: 291px;" />
+<pre>
+<strong>Input:</strong> root = [1,2,2,3,4,4,3]
+<strong>Output:</strong> true
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/symtree2.jpg" style="width: 308px; height: 258px;" />
+<pre>
+<strong>Input:</strong> root = [1,2,2,null,3,null,3]
+<strong>Output:</strong> false
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[1, 1000]</code>.</li>
+	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
+</ul>
+
+<p>&nbsp;</p>
+<strong>Follow up:</strong> Could you solve it both recursively and iteratively?