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";
        string s = "";
        for (int i = 7; i >= 0; --i) {
            int x = (num >> (4 * i)) & 0xf;
            if (s.size() > 0 || x != 0) {
                char c = x < 10 ? (char) (x + '0') : (char) (x - 10 + 'a');
                s += c;
            }
        }
        return s;
    }
};