204\. Count Primes

Medium

Given an integer `n`, return _the number of prime numbers that are strictly less than_ `n`.

**Example 1:**

**Input:** n = 10

**Output:** 4

**Explanation:** There are 4 prime numbers less than 10, they are 2, 3, 5, 7. 

**Example 2:**

**Input:** n = 0

**Output:** 0 

**Example 3:**

**Input:** n = 1

**Output:** 0 

**Constraints:**

*   <code>0 <= n <= 5 * 10<sup>6</sup></code>