Count Primes

Description:

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

public class Solution {
    public int countPrimes(int n) {
        if(n <= 2) return 0;
        boolean[] arr = new boolean[n];//use false to represent prime numbers
        //Actually here only use 1 ~ n-1
        int sqr = (int)Math.sqrt(n - 1);
        for(int i = 2; i <= sqr; i ++){
            if(!arr[i]){
                for(int j = i * i; j < n; j += i){
                    arr[j] = true;
                }
            }
        }
        int count = 0;
        for(int i = 2; i < n; i ++){
            if(!arr[i]) count ++;
        }
        return count;
    }
}

Another Solution:

public class Solution {
    public int countPrimes(int n) {
        if(n < 3) return 0;
        HashSet<Integer> primes = new HashSet<Integer> ();
        for(int i = 2; i < n; i ++){
            if(primes.isEmpty()){
                primes.add(i);
            }else{
                Iterator it = primes.iterator();
                boolean isPrime = true;
                while(it.hasNext()){
                    Integer tmp = (Integer) it.next();
                    if(i % tmp == 0) isPrime = false;
                }
                if(isPrime) primes.add(i);
            }
        }
        return primes.size();
    }
}

This one return Time Limit Exceeded for case: 499979