Added README.md file for Binary Tree Paths

Author: Odonata971

257-binary-tree-paths/README.md

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+<h2><a href="https://leetcode.com/problems/binary-tree-paths">Binary Tree Paths</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>all root-to-leaf paths in <strong>any order</strong></em>.</p>
+
+<p>A <strong>leaf</strong> is a node with no children.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/03/12/paths-tree.jpg" style="width: 207px; height: 293px;" />
+<pre>
+<strong>Input:</strong> root = [1,2,3,null,5]
+<strong>Output:</strong> [&quot;1-&gt;2-&gt;5&quot;,&quot;1-&gt;3&quot;]
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [1]
+<strong>Output:</strong> [&quot;1&quot;]
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+<li>The number of nodes in the tree is in the range <code>[1, 100]</code>.</li>
+<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
+</ul>