Added README.md file for Number of Stable Subsequences

Co-authored-by: Joshua Dierickse <joshua.dierickse@gmail.com>

Author: 2 people

4027-number-of-stable-subsequences/README.md

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+<h2><a href="https://leetcode.com/problems/number-of-stable-subsequences">Number of Stable Subsequences</a></h2> <img src='https://img.shields.io/badge/Difficulty-Hard-red' alt='Difficulty: Hard' /><hr><p>You are given an integer array <code>nums</code>.</p>
+
+<p>A <strong><span data-keyword="subsequence-array-nonempty">subsequence</span></strong> is <strong>stable</strong> if it does not contain <strong>three consecutive</strong> elements with the <strong>same</strong> parity when the subsequence is read <strong>in order</strong> (i.e., consecutive <strong>inside the subsequence</strong>).</p>
+
+<p>Return the number of stable subsequences.</p>
+
+<p>Since the answer may be too large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<div class="example-block">
+<p><strong>Input:</strong> <span class="example-io">nums = [1,3,5]</span></p>
+
+<p><strong>Output:</strong> <span class="example-io">6</span></p>
+
+<p><strong>Explanation:</strong></p>
+
+<ul>
+	<li>Stable subsequences are <code>[1]</code>, <code>[3]</code>, <code>[5]</code>, <code>[1, 3]</code>, <code>[1, 5]</code>, and <code>[3, 5]</code>.</li>
+	<li>Subsequence <code>[1, 3, 5]</code> is not stable because it contains three consecutive odd numbers. Thus, the answer is 6.</li>
+</ul>
+</div>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<div class="example-block">
+<p><strong>Input:</strong> <span class="example-io">nums = </span>[2,3,4,2]</p>
+
+<p><strong>Output:</strong> <span class="example-io">14</span></p>
+
+<p><strong>Explanation:</strong></p>
+
+<ul>
+	<li>The only subsequence that is not stable is <code>[2, 4, 2]</code>, which contains three consecutive even numbers.</li>
+	<li>All other subsequences are stable. Thus, the answer is 14.</li>
+</ul>
+</div>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>1 &lt;= nums[i] &lt;= 10<sup>​​​​​​​5</sup></code></li>
+</ul>