2787: 2014J-3 素数筛

#include <iostream>  
using namespace std;  
const int SIZE = 100;  
int main(){  
    int p[SIZE];  
    int n, tot, i, cn;  
    tot = 0;  
    cin >> n;  
    for ( i = 1; i <= n; i++ )  
        p[i] = 1;  
    for ( i = 2; i <= n; i++ ){  
        if ( p[i] == 1 )  
            tot++;  
        cn = i * 2;  
        while (cn <= n){  
            p[cn] = 0;  
            cn += i;  
        }  
    }  
    cout << tot << endl;  
    return(0);  
}