Added README.md file for Construct Binary Tree from Inorder and Postorder Traversal

Author: sayandip20

106-construct-binary-tree-from-inorder-and-postorder-traversal/README.md

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+<h2><a href="https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal">Construct Binary Tree from Inorder and Postorder Traversal</a></h2> <img src='https://img.shields.io/badge/Difficulty-Medium-orange' alt='Difficulty: Medium' /><hr><p>Given two integer arrays <code>inorder</code> and <code>postorder</code> where <code>inorder</code> is the inorder traversal of a binary tree and <code>postorder</code> is the postorder traversal of the same tree, construct and return <em>the binary tree</em>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/tree.jpg" style="width: 277px; height: 302px;" />
+<pre>
+<strong>Input:</strong> inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
+<strong>Output:</strong> [3,9,20,null,null,15,7]
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> inorder = [-1], postorder = [-1]
+<strong>Output:</strong> [-1]
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= inorder.length &lt;= 3000</code></li>
+	<li><code>postorder.length == inorder.length</code></li>
+	<li><code>-3000 &lt;= inorder[i], postorder[i] &lt;= 3000</code></li>
+	<li><code>inorder</code> and <code>postorder</code> consist of <strong>unique</strong> values.</li>
+	<li>Each value of <code>postorder</code> also appears in <code>inorder</code>.</li>
+	<li><code>inorder</code> is <strong>guaranteed</strong> to be the inorder traversal of the tree.</li>
+	<li><code>postorder</code> is <strong>guaranteed</strong> to be the postorder traversal of the tree.</li>
+</ul>