204_count_primes.java

class Solution {
    public int countPrimes(int n) {
        int[] arr = new int[n];
        if (n > 1) {
            arr[0] = 1;
            arr[1] = 1;
        } else {
            return 0;
        }
        for (int i = 2; i < Math.sqrt(n); i++) {
            if (arr[i] == 0) {
                // `j = i
                // for (int j = 2; i * j < n; j++) {
                for (int j = i; i * j < n; j++) {
                    arr[i*j] = 1;
                }
            }
        }
        int ansCount = 0;
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == 0) {
                ansCount++;
            }
        }
        return ansCount;
    }
}

/**
 * This is my personal record of solving Leetcode Problems. 
 * If you have any questions, please discuss them in [Issues](https://github.com/mengxinayan/leetcode/issues).
 * Copyright (C) 2020-2022  mengxinayan
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */