Happy Number
<p>Write an algorithm to determine if a number <code>n</code> is happy.</p>

<p>A <strong>happy number</strong> is a number defined by the following process:</p>

<ul>
	<li>Starting with any positive integer, replace the number by the sum of the squares of its digits.</li>
	<li>Repeat the process until the number equals 1 (where it will stay), or it <strong>loops endlessly in a cycle</strong> which does not include 1.</li>
	<li>Those numbers for which this process <strong>ends in 1</strong> are happy.</li>
</ul>

<p>Return <code>true</code> <em>if</em> <code>n</code> <em>is a happy number, and</em> <code>false</code> <em>if not</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 19
<strong>Output:</strong> true
<strong>Explanation:</strong>
1<sup>2</sup> + 9<sup>2</sup> = 82
8<sup>2</sup> + 2<sup>2</sup> = 68
6<sup>2</sup> + 8<sup>2</sup> = 100
1<sup>2</sup> + 0<sup>2</sup> + 0<sup>2</sup> = 1
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 2
<strong>Output:</strong> false
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 2<sup>31</sup> - 1</code></li>
</ul>
Hash Table; Math; Two Pointers
function isHappy(n: number): boolean {
    const getNext = (n: number) => {
        let res = 0;
        while (n !== 0) {
            res += (n % 10) ** 2;
            n = Math.floor(n / 10);
        }
        return res;
    };
    const set = new Set();
    while (n !== 1) {
        const next = getNext(n);
        if (set.has(next)) {
            return false;
        }
        set.add(next);
        n = next;
    }
    return true;
}
