计数质数

输入：n = 10
输出：4
解释：小于 10 的质数一共有 4 个, 它们是 2, 3, 5, 7 。

package com.apesblog.day_20221212;

/**
 * @author ZhangYuShun
 * @since 2022/12/12
 * 给定整数 n ，返回 所有小于非负整数 n 的质数的数量 。
 */

public class PrimeNumber {

    public static void main(String[] args) {
        int n = 499979;
        int count = countPrimeByFor(n);
        System.out.println(count);

        count = countPrimeByEratosthenes(n);
        System.out.println(count);
    }

    private static int countPrimeByEratosthenes(int n) {
        boolean[] isPrime = new boolean[n];
        for (int i = 0; i < n; i++) {
            isPrime[i] = true;
        }
        int count = 0;
        for (int i = 2; i < n; i++) {
            if (isPrime[i]) {
                count++;
                if ((long) i * i < n) {
                    for (int j = i * i; j < n; j += i) {
                        isPrime[j] = false;
                    }
                }
            }
        }
        return count;
    }

    private static int countPrimeByFor(int n) {
        int count = 0;
        for (int i = 2; i < n; i++) {
            if (isPrime(i)) {
                count++;
            }
        }
        return count;
    }

    private static boolean isPrime(int x) {
        for (int i = 2; i * i <= x; i++) {
            if (x % i == 0) {
                return false;
            }
        }
        return true;
    }
}