《算法竞赛入门经典》上的例题。 解题技巧蕴涵的东西不太懂，于是作篇记录。

The most efficient way to find every prime number no more than a specific number *N* is called **sieve of Eratosthenes**, or *sieve* for short. it has almost \(O(N)\) time complexity which can be obviously seen in the following introduction.