204 Count Prime

Time : O(n * sqrt(n))

class Solution {
    public int countPrimes(int n) {
        int count = 0;
        for (int i = 2; i < n; i++) {
            if(isPrime(i)) {
                count++;
            }
        }
        return count;
    }
    public boolean isPrime(int n) {
        for (int i = 2; i <= Math.sqrt(n); i++) {
            if (n % i == 0) {
                return false;
            }
        }
        return true;
    }
}

厄拉多塞筛法，求一组质数，时间复杂度仅有O(nloglogn)

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