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

6
8

 

Sample Output

Case 1:
1 4 3 2 5 6
1 6 5 2 3 4

Case 2:
1 2 3 8 5 6 7 4
1 2 5 8 3 4 7 6
1 4 7 6 5 8 3 2
1 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 long
using 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 long
using 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;
}