hdu 1016 Prime Ring Problem（素数环，超时）

A ring is compose of n circles as shown in diagram. Put natural number 1, 2, ..., n into each circle separately, and the sum of numbers in two adjacent circles should be a prime.

Note: the number of first circle should always be 1.

 

Input

n (0 < n < 20).

 

Output

The output format is shown as sample below. Each row represents a series of circle numbers in the ring beginning from 1 clockwisely and anticlockwisely. The order of numbers must satisfy the above requirements. Print solutions in lexicographical order.

You are to write a program that completes above process.

Print a blank line after each case.

 

Sample Input

68

 

Sample Output

Case 1:1 4 3 2 5 61 6 5 2 3 4Case 2:1 2 3 8 5 6 7 41 2 5 8 3 4 7 61 4 7 6 5 8 3 21 6 7 4 3 8 5 2

#include<iostream>#include<algorithm>#include<string>#include<string.h>#include<map>#include<vector>#include<cmath>#include<stdlib.h>#include<cstdio>#define ll long longusing namespace std;int n;int isprime(int n){    if(n==2||n==3||n==5||n==7||n==11||n==13||n==17||n==19||n==23||n==29||n==31||n==37)        return 1;    return 0;}int vis[22];int s[22];void dfs(int p){  //第p层     if(p==n){        if(isprime(s[0]+s[n-1])){            for(int i=0;i<n-1;++i)                printf("%d ",s[i]);            printf("%d\n",s[n-1]);        }        return;    }    for(int i=2;i<=n;++i){        if(isprime(i+s[p-1])&&vis[i]==0){            s[p]=i;            vis[i]=1;            dfs(p+1);            vis[i]=0;        }    }}int main(){int cnt=0;while(cin>>n){   //想不通为什么这里用scanf就错。。。memset(vis,0,sizeof(vis));printf("Case %d:\n",++cnt);        s[0]=1;        dfs(1);        printf("\n");    }    return 0;}

原本超时代码：

#include<iostream>#include<algorithm>#include<string>#include<string.h>#include<map>#include<vector>#include<cmath>#include<stdlib.h>#include<cstdio>#define ll long longusing namespace std;int n;int isprime(int n){if(n==2||n==3||n==5||n==7||n==11||n==13||n==17||n==19||n==23||n==31||n==37)return 1;return 0;}int v[22];int s[22];void dfs(int p,int q){s[q]=p;if(q==n-1){if(!isprime(s[0]+s[n-1]))return;for(int i=0;i<n-1;++i){printf("%d ",s[i]);}printf("%d\n",s[n-1]);return;}for(int i=1;i<=n;++i){if(v[i]==0&&isprime(p+i)){v[i]=1;dfs(i,q+1);v[i]=0;}}}int main(){int cnt=0;while(scanf("%d",&n)&&n!=EOF){memset(v,0,sizeof(v));v[1]=1;printf("Case %d:\n",++cnt);dfs(1,0);printf("\n");}return 0;}