204. Count Primes 

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class Solution {
    public int countPrimes(int n) {
        boolean[] primes = new boolean[n];
        Arrays.fill(primes, true);
        for (int i = 2; i * i < n; i++) {
            if (!primes[i]) {
                continue;
            }
            for (int j = i * i; j < n; j += i) {
                primes[j] = false;
            }
        }

        int count = 0;
        // 注意 1 不是质数，因为它只能被 1 个数整除，现代质数定义为必须被 1 和自身这两个不同的数字整除
        for (int i = 2; i < primes.length; i++) {
            if (primes[i]) {
                count++;
            }
        }
        return count;
    }
}

References

204. Count Primes