CountPrimes.java

package leetcode;

/**
 * Project Name : Leetcode
 * Package Name : leetcode
 * File Name : CountPrimes
 * Creator : Edward
 * Date : Nov, 2017
 * Description : 204. Count Primes
 */
public class CountPrimes {
    /**
     * 厄拉多塞筛法，求一组质数，时间复杂度仅有O(nloglogn)
     * 如果从1到n-1分别判断质数，时间复杂度为O(nsqrt(n))）
     * 1   2  3  4  5  6  7  8  9 10
     * 11 12 13 14 15 16 17 18 18 20
     *
     * @param n
     * @return
     */
    public int countPrimes(int n) {
        boolean[] notPrime = new boolean[n];
        int res = 0;
        for (int i = 2; i < n; i++) {
            if (notPrime[i] == false) {
                res++;
                for (int j = 2; i * j < n; j++) {
                    notPrime[i * j] = true;
                }
            }
        }
        return res;
    }
}