求质数

public static int countPrimes(int n) {    /**     * optimize the code, the non-prime numbers begin at p*p     */    int count = 0;    int[] store = new int[n+1];    for(int i=2;i<n;i++){    if(store[i]!=-1){    count++;    store[i] = 0;    for(int j=i*i;0<j && j<=n;j+=i){    store[j] = -1;    }    }    }        return count;}