poj 3518 Prime Gap

 1 #include<cmath>
 2 #include<cstdio>
 3 #include<algorithm>
 4 #include<iostream>
 5 #include<string>
 6 #include<cstring>
 7 #include<vector>
 8 using namespace std;
 9 #define max 100000
10 int prime[max+5];
11 //bool vis[max+5];
12 void get_prime(int n){
13     //memset(vis,false,sizeof(vis));
14     prime[0]=2;
15     prime[1]=2;
16     int i=3,index=2;
17     for(;index<=n;i+=2){
18         //cout<<i<<endl;
19         //cout<<prime[index-1]<<endl;
20         int half=sqrt(i*1.0)+1;
21         int j=1;
22         while(j<index&&half>prime[j]){//不知道为什么j<index可以去掉。去掉也是对的。
23             //cout<<i<<" "<<j<<" "<<prime[j]<<endl;
24             if(i%prime[j]==0){
25                 break;
26             }
27             j++;
28         }
29         if(half<=prime[j]){
30             prime[index++]=i;
31         }
32     }
33     //cout<<prime[index-1]<<endl;
34 }
35 int main(){
36     int n;
37     get_prime(max);
38     while(cin>>n&&n){
39         int i=1;
40         while(prime[i]<n){
41             i++;
42         }
43         if(prime[i]==n){
44             cout<<0<<endl;
45         }
46         else{
47             cout<<prime[i]-prime[i-1]<<endl;
48         }
49     }
50     return 0;
51 }