405\. Convert a Number to Hexadecimal

Easy

Given an integer `num`, return _a string representing its hexadecimal representation_. For negative integers, [two’s complement](https://en.wikipedia.org/wiki/Two%27s_complement) method is used.

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.

**Note:** You are not allowed to use any built-in library method to directly solve this problem.

**Example 1:**

**Input:** num = 26

**Output:** "1a" 

**Example 2:**

**Input:** num = -1

**Output:** "ffffffff" 

**Constraints:**

*   <code>-2<sup>31</sup> <= num <= 2<sup>31</sup> - 1</code>