204. Count Primes

JIAKAOBO

LeetCode

感谢赞助!

• ㊗️
• 大家
• offer
• 多多！

Problem

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:

• $0 <= n <= 5 * 10^6$

Code

class Solution {
    public int countPrimes(int n) {
        boolean[] notPrime = new boolean[n];
        int res = 0;

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

        return res;
    }
}