Added README.md file for Flatten Binary Tree to Linked List

Author: sayandip20

114-flatten-binary-tree-to-linked-list/README.md

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+<h2><a href="https://leetcode.com/problems/flatten-binary-tree-to-linked-list">Flatten Binary Tree to Linked List</a></h2> <img src='https://img.shields.io/badge/Difficulty-Medium-orange' alt='Difficulty: Medium' /><hr><p>Given the <code>root</code> of a binary tree, flatten the tree into a &quot;linked list&quot;:</p>
+
+<ul>
+	<li>The &quot;linked list&quot; should use the same <code>TreeNode</code> class where the <code>right</code> child pointer points to the next node in the list and the <code>left</code> child pointer is always <code>null</code>.</li>
+	<li>The &quot;linked list&quot; should be in the same order as a <a href="https://en.wikipedia.org/wiki/Tree_traversal#Pre-order,_NLR" target="_blank"><strong>pre-order</strong><strong> traversal</strong></a> of the binary tree.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/01/14/flaten.jpg" style="width: 500px; height: 226px;" />
+<pre>
+<strong>Input:</strong> root = [1,2,5,3,4,null,6]
+<strong>Output:</strong> [1,null,2,null,3,null,4,null,5,null,6]
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = []
+<strong>Output:</strong> []
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [0]
+<strong>Output:</strong> [0]
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[0, 2000]</code>.</li>
+	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
+</ul>
+
+<p>&nbsp;</p>
+<strong>Follow up:</strong> Can you flatten the tree in-place (with <code>O(1)</code> extra space)?