204. Count Primes

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

Solution:

class Solution {
    public int countPrimes(int n) {
        boolean[] isPrime = new boolean[n];

        Arrays.fill(isPrime, true);

        for (int i = 2; i * i < n; i++){
            if (isPrime[i]){
                for (int j = i * i; j < n; j += i){
                    isPrime[j] = false;
                }
            }
        }

        int count = 0;
        for (int i = 2; i < n; i++){
            if (isPrime[i]){
                count++;
            }
        }

        return count;
    }
}