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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0201_0300.s0204_count_primes;

// #Medium #Top_Interview_Questions #Array #Math #Enumeration #Number_Theory
// #2022_06_28_Time_115_ms_(92.82%)_Space_46.8_MB_(85.39%)

/**
 * 204 - Count Primes\.
 *
 * Medium
 *
 * Given an integer `n`, return _the number of prime numbers that are strictly less than_ `n`.
 *
 * **Example 1:**
 *
 * **Input:** n = 10
 *
 * **Output:** 4
 *
 * **Explanation:** There are 4 prime numbers less than 10, they are 2, 3, 5, 7. 
 *
 * **Example 2:**
 *
 * **Input:** n = 0
 *
 * **Output:** 0 
 *
 * **Example 3:**
 *
 * **Input:** n = 1
 *
 * **Output:** 0 
 *
 * **Constraints:**
 *
 * *   0 <= n <= 5 * 10⁶
**/
public class Solution {
    public int countPrimes(int n) {
        boolean[] isprime = new boolean[n];
        int count = 0;
        for (int i = 2; i * i <= n; i++) {
            if (!isprime[i]) {
                for (int j = i * 2; j < n; j += i) {
                    isprime[j] = true;
                }
            }
        }
        for (int i = 2; i < isprime.length; i++) {
            if (!isprime[i]) {
                count++;
            }
        }
        return count;
    }
}