【Leetcode】Count Primes

题目链接：https://leetcode.com/problems/count-primes/

题目：

Description:

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

思路：

埃拉托色尼选筛法

算法：

public int countPrimes(int n) {  
        boolean c[] = new boolean[n];  
        for (int i = 2; i * i < n; i++) {  
            if(!c[i]){  
                for (int j = i+i; j < n; j += i) {  
                    if(!c[j])  
                        c[j] = true;  
                }  
            }  
        }  
        int count = 0;  
        for (int i = 2; i < n; i++) {  
            if (!c[i])  
                count++;  
        }  
        return count;  
    }