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
impl Solution {
    pub fn convert_to_base7(mut num: i32) -> String {
        if num == 0 {
            return String::from("0");
        }
        let mut res = String::new();
        let is_minus = num < 0;
        if is_minus {
            num = -num;
        }
        while num != 0 {
            res.push_str((num % 7).to_string().as_str());
            num /= 7;
        }
        if is_minus {
            res.push('-');
        }
        res.chars().rev().collect()
    }
}
