Base 7
<p>Given an integer <code>num</code>, return <em>a string of its <strong>base 7</strong> representation</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> num = 100
<strong>Output:</strong> "202"
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> num = -7
<strong>Output:</strong> "-10"
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>-10<sup>7</sup> &lt;= num &lt;= 10<sup>7</sup></code></li>
</ul>
Math; String
function convertToBase7(num: number): string {
    if (num == 0) {
        return '0';
    }
    let res = '';
    const isMinus = num < 0;
    if (isMinus) {
        num = -num;
    }
    while (num != 0) {
        const r = num % 7;
        res = r + res;
        num = (num - r) / 7;
    }
    return isMinus ? '-' + res : res;
}
