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

Example:

Input: 10Output: 4Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.

难度：easy

题目：统计小于非负整数n的所有素数。

思路：参考素数筛选法。

Runtime: 11 ms, faster than 94.69% of Java online submissions for Count Primes.

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