POJ 3006 Dirichlet's Theorem on Arithmetic Progressions 素数

题意：给出一列等差数列。等差数列的起点a与公差d互素。求数列的第n个素数。

#include <cmath>
#include <iostream>
using namespace std;

int Prime[1000000];

int main()
{
int i, j;
memset(Prime,-1,sizeof(Prime));
Prime[0] = Prime[1] = 0;
for ( i = 2; i <= 1000; i++ )
{
if ( Prime[i] )
for ( j = 2; i * j <= 1000000; j++ )
Prime[i*j] = 0;
}

int a, d, n, counter;
while ( cin >> a >> d >> n )
{
counter = 0;
if ( a == 0 && d == 0 && n == 0 ) break;
for ( i = a; counter != n; i += d )
if ( Prime[i] )
counter++;
cout << i - d << endl;
}
return 0;
}