素数对猜想 (20)

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
int main(void)
{
    int n;
    scanf("%d",&n);

    //筛法，很适合找出前n个数中的所有素数
    bool *a = (bool*)malloc((n + 1)*sizeof(bool));
    memset(a, true, n + 1);
    a[0] = a[1] = false;
    for (int i = 2; i <= sqrt(n) + 1; i++){
        if (a[i] == false){
            continue;
        }
        for (int j = 2; i*j <= n; j++){
            a[i*j] = false;
        }
    }

    int count = 0;
    for (int i = 3; i <= n - 2; i += 2){
        if (a[i] == true && a[i + 2] == true){
            count++;
        }
    }
    printf("%d",count);
    return 0;
}