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