HDU : 2138 (How many prime numbers)


// http://acm.hdu.edu.cn/showproblem.php?pid=2138

// @BEGIN_OF_SOURCE_CODE

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <string>
#include <cctype>
#include <stack>
#include <queue>
#include <list>
#include <vector>
#include <map>
#include <sstream>
#include <utility>
#include <set>
#include <math.h>
#define pi acos(-1.0)
#define N 1000000
using namespace std;

bool mark [N];
vector <int> primeList;

void sieve ()
{
    memset (mark, true, sizeof (mark));

    mark [0] = mark [1] = false;

    for ( int i = 4; i < N; i += 2 )
        mark [i] = false;

    for ( int i = 3; i * i <= N; i += 2 ) {
        if ( mark [i] ) {
            for ( int j = i * i; j < N; j += 2 * i )
                mark [j] = false;
        }
    }

    primeList.clear ();
    primeList.push_back (2);

    for ( int i = 3; i < N; i += 2 ) {
        if ( mark [i] )
            primeList.push_back (i);
    }

    //printf ("%d\n", primeList.size ());
}

bool isPrime (long long x)
{
    if ( x < N )
        return mark [x];

    int length = (int) sqrt (x);

    for ( int i = 0; primeList [i] <= length; i++ ) {
        if ( x % primeList [i] == 0 )
            return false;
    }

    return true;
}


int main ()
{
    sieve ();

    int n;

    while ( scanf ("%d", &n) != EOF ) {
        long long input;
        int cnt = 0;

        for ( int i = 0; i < n; i++ ) {
            cin >> input;

            if ( isPrime (input) )
                cnt++;
        }

        printf ("%d\n", cnt);
    }

    return 0;
}

// @END_OF_SOURCE_CODE
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