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2699-count-the-number-of-fair-pairs/README.md

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+<h2><a href="https://leetcode.com/problems/count-the-number-of-fair-pairs/">2699. Count the Number of Fair Pairs</a></h2><h3>Medium</h3><hr><p>Given a <strong>0-indexed</strong> integer array <code>nums</code> of size <code>n</code> and two integers <code>lower</code> and <code>upper</code>, return <em>the number of fair pairs</em>.</p>
+
+<p>A pair <code>(i, j)</code> is <b>fair </b>if:</p>
+
+<ul>
+	<li><code>0 &lt;= i &lt; j &lt; n</code>, and</li>
+	<li><code>lower &lt;= nums[i] + nums[j] &lt;= upper</code></li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [0,1,7,4,4,5], lower = 3, upper = 6
+<strong>Output:</strong> 6
+<strong>Explanation:</strong> There are 6 fair pairs: (0,3), (0,4), (0,5), (1,3), (1,4), and (1,5).
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [1,7,9,2,5], lower = 11, upper = 11
+<strong>Output:</strong> 1
+<strong>Explanation:</strong> There is a single fair pair: (2,3).
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>nums.length == n</code></li>
+	<li><code><font face="monospace">-10<sup>9</sup></font>&nbsp;&lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
+	<li><code><font face="monospace">-10<sup>9</sup>&nbsp;&lt;= lower &lt;= upper &lt;= 10<sup>9</sup></font></code></li>
+</ul>