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;

}

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hdu 素数环

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