POJ-2689 Prime Distance(线性筛法)

 1 #include<iostream>
 2 #include<cstring>
 3 #include<cstdio>
 4 #define N 50000
 5 #define len 1000000
 6 #define inf 0x7fffffff
 7 using namespace std;
 8 typedef long long LL;
 9 bool isprime[N+5];
10 LL prime[N],cnt;
11 bool res[len+5];
12 void init(){
13     LL i,j;
14     cnt=0;
15     memset(isprime,true,sizeof(isprime));
16     for (i=2;i<=N;i++){
17         if (isprime[i]) prime[cnt++]=i;
18         for (j=0;j<cnt && i*prime[j]<=N;j++){
19             isprime[i*prime[j]]=false;
20             if (i%prime[j]==0) break;
21         }
22     }
23 }
24 int main(){
25     freopen ("prime.in","r",stdin);
26     freopen ("prime.out","w",stdout);
27     LL L,U,i,j,k,min,max,s,t;
28     init();
29     while(scanf("%lld%lld",&L,&U)!=EOF){
30         memset(res,0,sizeof(res));
31         for(i=0;i<cnt;i++){
32             s=(L-1)/prime[i]+1;
33             t=U/prime[i];
34             for(j=s;j<=t;j++)
35                 if(j>1)
36                     res[j*prime[i]-L]=true;
37         }
38         k=-1,min=inf,max=-1;
39         LL dis,m1,m2;
40         for(i=0;i<=U-L;i++){
41             if(!res[i]){
42                 if(k!=-1){
43                     dis=i-k;
44                     if(dis>max){
45                         max=dis;
46                         m1=i;
47                     }
48                     if(dis<min){
49                         min=dis;
50                         m2=i;
51                     }
52                 }
53                 if(i+L!=1)//注意 1 的时候特殊判断 1 不是素数
54                     k=i;
55             }
56         }
57         if(max==-1)
58             printf("There are no adjacent primes.\n");
59         else{
60             printf("%lld,%lld are closest, %lld,%lld are most distant.\n",m2-
61             min+L,m2+L,m1-max+L,m1+L);
62         }
63     }
64     return 0;
65 }