LeetCode - 204. Count Primes

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.

统计小于n的素数个数，直接枚举会超时。这里使用素数筛

class Solution {
public:
    int countPrimes(int n) {
        int *table = new int[n];
        table[2] = 0;
        
        int count = 0;
        for( int i = 2; i < n; i++ ) {
            if( table[i] == 0 ) {
                count++;
                for( int j = i * 2; j < n; j += i ) {
                    table[j] = 1;
                }
            }
        }
        
        return count;
    }
};