504\. Base 7

Easy

Given an integer `num`, return _a string of its **base 7** representation_.

**Example 1:**

**Input:** num = 100

**Output:** "202"

**Example 2:**

**Input:** num = -7

**Output:** "-10"

**Constraints:**

*   <code>-10<sup>7</sup> <= num <= 10<sup>7</sup></code>