51nod 1181 质数中的质数（质数筛法）

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题目链接： [

http://www.51nod.com/onlineJudge/questionCode.html#!problemId=1181
](http://www.51nod.com/onlineJudge/questionCode.html#!problemId=1181)

代码：

#include <stdio.h>  
#include <iostream>  
#include <string.h>  
#include <algorithm>  
#include <math.h>  
#include <stack>  
#include <queue>  

using namespace std;
const int MAXN = 10000010;

bool com[MAXN];
int primes, prime[MAXN / 10];
int num[MAXN];

void solve(int n)
{
    primes = 0;
    memset(com, false, sizeof(com));
    com[0] = com[1] = true;
    for (int i = 2; i <= n; ++i)
    {
        if (!com[i])
        {
            prime[++primes] = i;
            num[i] = primes;
        }
        for (int j = 1; j <= primes && i*prime[j] <= n; ++j)
        {
            com[i*prime[j]] = true;
            if (!(i % prime[j]))
                break;
        }
    }
}

int main()
{
    solve(1000010);
    int n;
    while (cin >> n)
    {
        while (!(!com[n] && !com[num[n]]))
        n++;
        cout << n << endl;
    }
    return 0;
}