204. Count Primes

QUESTION:

Description:

Count the number of prime numbers less than a non-negative number, n.

Credits: Special thanks to @mithmatt for adding this problem and creating all test cases.

EXPLANATION:

具体的算法可以查看这个视频，其实就是筛选法的一种变形。

1.首先2是素数，所以可以把2的所有倍数都去除。

2.紧接着找到下一位，下一位就是3，同时去除3的倍数。

3.如此进行到

SOLUTION:

public class Solution {
    public int countPrimes(int n) {
        if (n < 3)
            return 0;
            
        boolean[] f = new boolean[n];
        int count = n / 2;
        for (int i = 3; i * i < n; i += 2) {
            if (f[i])
                continue;
            
            for (int j = i * i; j < n; j += 2 * i) {
                if (!f[j]) {
                    --count;
                    f[j] = true;
                }
            }
        }
        return count;
    }
    
}
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