Count Primes

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

答案

class Solution {
    public int countPrimes(int n) {
        if(n <= 1) return 0;
        int ans = 0;
        // 0, 1 .... n - 1
        boolean[] primes = new boolean[n];        
        primes[1] = false;
        
        for(int i = 2; i < primes.length; i++) 
            primes[i] = true;
        
        for(int i = 2; i <= Math.sqrt(n - 1); i++) {
            if(primes[i]) {
                for(int j = 2; i * j <= (n - 1); j++) {
                    primes[i * j] = false;
                }   
            }
        }
        
        for(int i = 1; i < primes.length; i++) 
            if(primes[i] == true) ans++;
        
        return ans;
    }
}