HDU 1016 Prime Ring Problem（DFS入门）

Prime Ring Problem

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 36781    Accepted Submission(s): 16202

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
  
  

题意：输入一个数字，输出一个素数环，要求每个相邻的数字和为质数，第一个数固定为1

代码如下：

#include <stdio.h>
#include <string.h>
int ans[25];
int a[25];
int n;
int f[35];

void fun()
{
	int i,j;
	memset(f,0,sizeof(f));
	
	for(i=2;i<=30;i++)
	{
		for(j=2*i;j<=30;j+=i)
			f[j]=1;
	}
}

void dfs(int t)
{
	int i;
	if(t>n)
	{
		if(f[ans[n]+1]==0)
		{
			printf("1");
			for(i=2;i<=n;i++)
				printf(" %d",ans[i]);
			putchar('\n');
		}
		return ;
	}
	
	for(i=2;i<=n;i++)
	{
		if(a[i]==0 && f[ans[t-1]+i]==0)
		{
			a[i]=1;
			ans[t]=i;
			dfs(t+1);
			a[i]=0;
		}
	}
}

int main()
{
	int T=0;
	fun();
	while(~scanf("%d",&n))
	{
		T++;
		printf("Case %d:\n",T);
		memset(a,0,sizeof(a));
		
		a[1]=1;
		ans[1]=1;		
		dfs(2);
		putchar('\n');
	}
	
	return 0;
}