Convert a Number to Hexadecimal
<p>Given a 32-bit integer <code>num</code>, return <em>a string representing its hexadecimal representation</em>. For negative integers, <a href="https://en.wikipedia.org/wiki/Two%27s_complement" target="_blank">two&rsquo;s complement</a> method is used.</p>

<p>All the letters in the answer string should be lowercase characters, and there should not be any leading zeros in the answer except for the zero itself.</p>

<p><strong>Note:&nbsp;</strong>You are not allowed to use any built-in library method to directly solve this problem.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> num = 26
<strong>Output:</strong> "1a"
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> num = -1
<strong>Output:</strong> "ffffffff"
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>-2<sup>31</sup> &lt;= num &lt;= 2<sup>31</sup> - 1</code></li>
</ul>
Bit Manipulation; Math; String
func toHex(num int) string {
	if num == 0 {
		return "0"
	}
	sb := &strings.Builder{}
	for i := 7; i >= 0; i-- {
		x := num >> (4 * i) & 0xf
		if x > 0 || sb.Len() > 0 {
			var c byte
			if x < 10 {
				c = '0' + byte(x)
			} else {
				c = 'a' + byte(x-10)
			}
			sb.WriteByte(c)
		}
	}
	return sb.String()
}