Added README.md file for N-th Tribonacci Number

Author: achuatchaya

1236-n-th-tribonacci-number/README.md

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+<h2><a href="https://leetcode.com/problems/n-th-tribonacci-number">N-th Tribonacci Number</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>The Tribonacci sequence T<sub>n</sub> is defined as follows:&nbsp;</p>
+
+<p>T<sub>0</sub> = 0, T<sub>1</sub> = 1, T<sub>2</sub> = 1, and T<sub>n+3</sub> = T<sub>n</sub> + T<sub>n+1</sub> + T<sub>n+2</sub> for n &gt;= 0.</p>
+
+<p>Given <code>n</code>, return the value of T<sub>n</sub>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> n = 4
+<strong>Output:</strong> 4
+<strong>Explanation:</strong>
+T_3 = 0 + 1 + 1 = 2
+T_4 = 1 + 1 + 2 = 4
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> n = 25
+<strong>Output:</strong> 1389537
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>0 &lt;= n &lt;= 37</code></li>
+	<li>The answer is guaranteed to fit within a 32-bit integer, ie. <code>answer &lt;= 2^31 - 1</code>.</li>
+</ul>