hdu 素数环

算法：搜索

题意：相邻的两个数之和是素数，别忘了最后一个，和第一个

Problem Description
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

代码：

#include <iostream>#include <string>#include <algorithm>#include <iomanip>#include <cstring>using namespace std;int a[30],b[45],n,m,c[30];void cmp(){   memset(b,1,sizeof(b));    b[1]=b[0]=0;for(int i=2;i<=22;i++){for(int j=2*i;j<45;j=j+i)    b[j]=0;}}void dfs(int x,int k){if(k==n&&b[c[0]+c[n-1]]){for(int i=0;i<n;i++){if(i)cout<<" ";cout<<c[i];}cout<<endl;}else {for(int i=2;i<=n;i++){if(!a[i]&&b[i+c[k-1]]){a[i]=1;c[k]=i;dfs(n,k+1);a[i]=0;}}}} int main(){int i,j=0;cmp();while(cin>>n){for(i=1;i<=n;i++)   a[i]=0;c[0]=1;cout<<"Case "<<++j<<":"<<endl;dfs(n,1);cout<<endl;}return 0;}