1556. Thousand Separator

https://leetcode.com/problems/thousand-separator

Description

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"

Example 3:

**Input:** n = 123456789
**Output:** "123.456.789"

Example 4:

**Input:** n = 0
**Output:** "0"

Constraints:

• 0 <= n < 2^31

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