<p>Given an integer <code>n</code>, return <em>the number of prime numbers that are strictly less than</em> <code>n</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 10
<strong>Output:</strong> 4
<strong>Explanation:</strong> There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 0
<strong>Output:</strong> 0
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> 0
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= n &lt;= 5 * 10<sup>6</sup></code></li>
</ul>

import numpy as np

class Solution:
    def countPrimes(self, n: int) -> int:
        
        if n <= 2: return 0
        is_prime = [True]*n
        is_prime[0] = is_prime[1] = False
        
        for i in range(2, int(n**0.5)+1):
            if is_prime[i]:
                for j in range(i*i, n, i):
                    is_prime[j] = False
        return sum(is_prime)
        
#         [0 0 1 1 0 1 0 1 0 0]
        
#         if n <= 2:
#             return 0

#         # 모든 수를 소수로 가정하고 True로 설정
#         is_prime = [True] * n
#         is_prime[0] = is_prime[1] = False  # 0과 1은 소수가 아님

#         # 에라토스테네스의 체 알고리즘
#         print(n)
#         print(n**0.5)
#         for i in range(2, int(n ** 0.5) + 1):
#             print('i: %d'%(i))
#             if is_prime[i]:
#                 for j in range(i * i, n, i):
#                     print('\tj: %d'%(j))
#                     is_prime[j] = False
#                 print(is_prime)

#         # True로 남아있는 것들의 개수를 세기
#         return sum(is_prime)
        
# #         if n <= 1: return 0
        
# #         # Study
# #         n_arr = np.zeros(n); n_arr[0] = 1
# #         while 1:
# #             if all(n_arr!=0): break
# #             i = np.argwhere(n_arr==0)[0,0]
# #             k = i+1
# #             n_arr[i] -= 1
# #             n_arr[k*k-1::k] += 1 # [i+k::k]
# #         return sum(n_arr[:-1]==-1)
            
67456