Added README.md file for Number Complement

Author: achuatchaya

476-number-complement/README.md

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+<h2><a href="https://leetcode.com/problems/number-complement">Number Complement</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>The <strong>complement</strong> of an integer is the integer you get when you flip all the <code>0</code>&#39;s to <code>1</code>&#39;s and all the <code>1</code>&#39;s to <code>0</code>&#39;s in its binary representation.</p>
+
+<ul>
+	<li>For example, The integer <code>5</code> is <code>&quot;101&quot;</code> in binary and its <strong>complement</strong> is <code>&quot;010&quot;</code> which is the integer <code>2</code>.</li>
+</ul>
+
+<p>Given an integer <code>num</code>, return <em>its complement</em>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> num = 5
+<strong>Output:</strong> 2
+<strong>Explanation:</strong> The binary representation of 5 is 101 (no leading zero bits), and its complement is 010. So you need to output 2.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> num = 1
+<strong>Output:</strong> 0
+<strong>Explanation:</strong> The binary representation of 1 is 1 (no leading zero bits), and its complement is 0. So you need to output 0.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= num &lt; 2<sup>31</sup></code></li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Note:</strong> This question is the same as 1009: <a href="https://leetcode.com/problems/complement-of-base-10-integer/" target="_blank">https://leetcode.com/problems/complement-of-base-10-integer/</a></p>