素数筛法打表 //j=i<<1等价于 j=i*2，即j是i的两倍，而最后的j+=i，则表示下一个循环j是i的3倍，接着4倍。。。 //i的所有2~N倍数肯定都不是素数，因此将flag置为0，直到最后一位。

Largest prime factor Everybody knows any number can be combined by the prime number. Now, your task is telling me what position of the largest prime factor. The position of prime 2 is 1, prime 3 is 2, and prime 5 is 3, etc. Specially, LPF(1) = 0. Ea