204. Count Primes

还记得里扣而令四，一点点变化，要打印出来。
Count the number of prime numbers less than a non-negative number, n.

Example:

Input: 10
Output: 4
Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.



https://www.youtube.com/watch?v=Kwo2jkHOyPY

数学题

这个题， 开始的 判断一个数是不是prime 的方法 给弄错了

count the number of odd (less than n except 1) and 2;
check the odd is prime or not

The Sieve Of Eratosthenes



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