【POJ3006】Dirichlet's Theorem on Arithmetic Progressions（素数筛法）

简单的暴力筛法就可。

#include <iostream>
#include <cstring>
#include <cmath>
#include <cctype>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <numeric>
using namespace std;

const int N = 1000005, M = 500005;
bool is[N]; int prm[M];
int getprm (int n) {
    int i, j, k = 0;
    int s, e = (int)(sqrt (0.0 + n) + 1);
    memset (is, 1, sizeof(is));
    prm[k ++] = 2; is[0] = is[1] = 0;
    for (i = 4; i < n; i += 2) is[i] = 0;
    for (i = 3; i < e; i += 2) {
        if (is[i]) {
            prm[k ++] = i;
            for (s = i * 2, j = i * i; j < n; j += s) {
                is[j] = 0;
            }
        }
    }
    for (; i < n; i += 2) {
        if (is[i]) prm[k ++] = i;
    }
    return k;
}

int main () {
    ios :: sync_with_stdio(false);
    //freopen("out.txt", "w", stdout);
    getprm(1000000);
    /*for (int i = 0 ; i < 78499; ++ i) {
        cout << i  << " : " << prm[i] << endl;
    }*/
    long long a, d, n;
    long long cnt = 0, cur = 0;
    while (cin >> a >> d >> n) {
        if (a == 0 && d == 0 && n == 0) break;
        cnt = 0 ;
        while (1) {
            if (binary_search(prm, prm + 78499, a)) {
                cnt ++;
                if (cnt == n) break;
            }
            a += d;
            //cout << "step: " << cnt << " " << a << endl;
        }
        cout << a << endl;
    }
    return 0;
}