Count Primes

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