leetcode 204. Count Primes 计算质数的数目

Count the number of prime numbers less than a non-negative number, n. Example: Input: 10 Output: 4 Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7. 常规方法很容易超时，要使用高效的算法，这是google

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