N-th Tribonacci Number
<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>
Memoization; Math; Dynamic Programming
/**
 * @param {number} n
 * @return {number}
 */
var tribonacci = function (n) {
    let a = 0;
    let b = 1;
    let c = 1;
    while (n--) {
        let d = a + b + c;
        a = b;
        b = c;
        c = d;
    }
    return a;
};
