1556\. Thousand Separator

Easy

Given an integer `n`, add a dot (".") as the thousands separator and return it in string format.

**Example 1:**

**Input:** n = 987

**Output:** "987"

**Example 2:**

**Input:** n = 1234

**Output:** "1.234"

**Constraints:**

*   <code>0 <= n <= 2<sup>31</sup> - 1</code>