题目描述：
Count the number of prime numbers less than a non-negative number, n
Hint: The number n could be in the order of 100,000 to 5,000,000.
References:
How Many Primes Are There? (https://primes.utm.edu/howmany.html)
Sieve of Eratosthenes (http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)
题目大意：
统计小于非负整数n的素数的个数
提示：n的范围是100,000到5,000,000 ...