204. Count Primes

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

Example:

Input: 10
Output: 4
Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.


題意：找素數，找n以內有多少個素數
篩數法統計

class Solution {
    public int countPrimes(int n){
        
 
if (n == 0 || n == 1 || n == 2)
            return 0;
        int[] num = new int [n];
        for (int i = 0; i < n; i++) {
            num[i] = i;
        }

        for (int i = 2; i < n; i++) {
            if (num[i] != 0) {
                for (int j = 2; j*i < n; j ++)
                    num[j
 
*i] = 0;
            }
        }
        int sum = 0;
        for (int i = 0; i < n; i++) {
            if (num[i] != 0)
                sum ++;
        }
        return sum - 1;
    }
}

最快的算法應該是 6n那個，但是我不會，hhaha有興趣的小夥伴可以自學一下

[LeetCode] 204. Count Primes