Given an integer `n`, return `true` if `n` has exactly three positive divisors. Otherwise, return `false`.

An integer `m` is a divisor of `n` if there exists an integer `k` such that `n = k * m`.

Example 1:

    Input: n = 2
    Output: false
    Explantion: 2 has only two divisors: 1 and 2.

Example 2:

    Input: n = 4
    Output: true
    Explantion: 4 has three divisors: 1, 2, and 4.

Constraints:

- `1 <= n <= 104`