Added README.md file for Check If N and Its Double Exist

Author: kunavamshi

1468-check-if-n-and-its-double-exist/README.md

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+<h2><a href="https://leetcode.com/problems/check-if-n-and-its-double-exist">Check If N and Its Double Exist</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Given an array <code>arr</code> of integers, check if there exist two indices <code>i</code> and <code>j</code> such that :</p>
+
+<ul>
+	<li><code>i != j</code></li>
+	<li><code>0 &lt;= i, j &lt; arr.length</code></li>
+	<li><code>arr[i] == 2 * arr[j]</code></li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> arr = [10,2,5,3]
+<strong>Output:</strong> true
+<strong>Explanation:</strong> For i = 0 and j = 2, arr[i] == 10 == 2 * 5 == 2 * arr[j]
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> arr = [3,1,7,11]
+<strong>Output:</strong> false
+<strong>Explanation:</strong> There is no i and j that satisfy the conditions.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>2 &lt;= arr.length &lt;= 500</code></li>
+	<li><code>-10<sup>3</sup> &lt;= arr[i] &lt;= 10<sup>3</sup></code></li>
+</ul>