HDU 3792 素数打表

 1 #include<bits/stdc++.h>
 2 #define ll __int64
 3 #define mod 1e9+7
 4 #define PI acos(-1.0)
 5 #define bug(x) printf("%%%%%%%%%%%%%",x);
 6 #define inf 1e8
 7 using namespace std;
 8 int n;
 9 bool visit[100005];
10 int prime[100005];
11 int ans[100005];
12 void fun()
13 {
14     memset(visit, true, sizeof(visit));
15     int num = 0;
16     visit[1]=false;
17     for (int i = 2; i <= 100000; ++i)
18     {
19         if (visit[i] == true)
20         {
21             num++;
22             prime[num]=i;
23             //mp[i]=num;
24         }
25     for (int j=1;((j<=num)&&(i*prime[j]<= 100000));++j)
26         {
27             visit[i*prime[j]] =false;
28             if (i%prime[j]==0)break; //点睛之笔
29         }
30     }
31     int jishu=0;
32     for(int i=3;i<=100000;i++)
33     {
34         if(visit[i]&&visit[i-2])
35             jishu++;
36         ans[i]=jishu;
37     }
38     ans[0]=0;
39     ans[1]=0;
40     ans[2]=0;
41 }
42 int main()
43 {
44     fun();
45     while(scanf("%d",&n)!=EOF)
46     {
47         if(n<0)
48             break;
49         printf("%d\n",ans[n]);
50     }
51     return 0;
52 }