LeetCode-in-Java
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
Solution
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;
}
}