百度-双素数-c++

/*
 * DoubleNum.cpp
 *
 *  Created on: 2017年9月27日
 *      Author: Administrator
 */

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <map>
#include <string>
#include <vector>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <algorithm>
#include <unordered_map>
using namespace std;

bool p[1000010];

void creatTable() {
    memset(p, true, sizeof(p));
    p[0] = false; //0不是素数
    p[1] = false; //1不是素数
    for (int i = 2; i * i < 1000000; i++) {
        for (int j = 2; j * i < 1000000; j++) {
            p[j * i] = false; //筛掉2，3，……的倍数
        }
    }
}
int reverse(int num) { //倒置素数
    int *R = new int[1000000];
    int count = 0;
    while (num) {
        R[count++] = num % 10;
        num /= 10;
    }
    int out = 0;
    for (int i = 0; i < count; i++) {
        out = out * 10 + R[i];
    }
    delete R;
    return out;
}
int main() {
    creatTable();
    int k;
    while (cin >> k) {
        int count = 0;
        int result = -1;
        for (int i = 10; i < 1000000; i++) {
            if (p[i] && p[reverse(i)] && (i != reverse(i)))
                count++;
            if (count == k) {
                result = i;
                break;
            }
        }
        cout << result << endl;
    }
    return 0;
}

参考：
https://www.nowcoder.com/discuss/50105