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
class Solution {
    public String toHex(int num) {
        if (num == 0) {
            return "0";
        }
        StringBuilder sb = new StringBuilder();
        while (num != 0) {
            int x = num & 15;
            if (x < 10) {
                sb.append(x);
            } else {
                sb.append((char) (x - 10 + 'a'));
            }
            num >>>= 4;
        }
        return sb.reverse().toString();
    }
}