Count Primes

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

Solution:

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