leetcode/CountPrimes.java at master · chuyi2007/leetcode · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
public class Solution {
    public int countPrimes(int n) {
        if (n < 2) {
            return 0;
        }
        boolean[] isPrime = new boolean[n];
        Arrays.fill(isPrime, true);
        for (int i = 2; i < n; ++i) {
            if (isPrime[i]) {
                for (int j = 2; i * j < n; ++j) {
                    isPrime[i * j] = false;
                }
            }
        }

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