leetcode 204: Count Primes

Description:

Count the number of prime numbers less than a non-negative number, n

[思路]

素数不能被比它小的整数整除, 建一个boolean 数组, 从2开始, 把其倍数小于N的都删掉.

注意 inner loop从i开始, 比i小的会在以前就被check过.

[CODE]

public class Solution {
    public int countPrimes(int n) {
     //2,3,5,7,11,13,17 
     //20  5
     //init check  n
     boolean[] a = new boolean[n];
     for(int i=2; i*i