Prime Ring Problem prime ring problem

Let's take this question Kankan: Prime Ring Problem


just started this idea or circulation problems first, then thought ah recycling is not feasible, then it can only be recursive, but recursive how to do it? ?

Recursion

Recursion is looking for export recursive and recursive body it
recursive exports this question is to determine your last digit and the first digit (ie 1) and is not a prime number, then how to judge it?

void dfs(int num,int n)
{
	int i;
	if(num>=n+1&&flag[a[n]+1]==1)
	{
		for(i=1;i<n;i++)
		{
			printf("%d ",a[i]);
		}
		printf("%d\n",a[n]);
		return ;
	}
	else
	{
		for(i=2;i<=n;i++)
		{
			if(visit[i]==0&&flag[a[num-1]+i])
			{
				visit[i]=1;
				a[num]=i;
				dfs(num+1,n);
				visit[i]=0;
			}
		}return ;
	}
}

num represents the number of digits here I was about to discharge, that the first few numbers, n n is the input of our

Since we properly arranged into an array of numbers a, and start with an index 1, so a total of n digital requires n + 1, the final determination that the n-th digit, flag [a [n] +1 ] indicates the final determination and a first number and the number is not a prime number.
If so, then the output of the contents of the array a
, if not, begin to judge from the number 2 n-:
1. determining whether the accessed number is
2. This number is determined and an array in a digital one is prime and
if not and it has been visited and is a prime number, then this number is marked as visited, and stored in the array a, and then recursively to find a number, and finally remember to visit marking clear, because in the end will definitely visit again backtracking.

Code

#include<stdio.h>
int prime[13]={2,3,5,7,11,13,17,19,23,29,31,37,41};
int n,flag[40]={0},visit[21]={0},a[21]={0 };
void dfs(int num,int n)
{
	int i;
	if(num>=n+1&&flag[a[n]+1]==1)
	{
		for(i=1;i<n;i++)
		{
			printf("%d ",a[i]);
		}
		printf("%d\n",a[n]);
		return ;
	}
	else
	{
		for(i=2;i<=n;i++)
		{
			if(visit[i]==0&&flag[a[num-1]+i])
			{
				visit[i]=1;
				a[num]=i;
				dfs(num+1,n);
				visit[i]=0;
			}
		}return ;
	}
}
int main()
{
	int i,k=0,count=1;
	for(i=0;i<40;i++)
	{
		if(i==prime[k])
		{
			flag[i]=1;
			k++;
		}
	}
	while(scanf("%d",&n)!=EOF&&n!=0)
	{
		for(i=1;i<=n;i++)
		{
			a[i]=0;
			visit[i]=0;
		}
		a[1]=1;visit[1]=1;
		printf("Case %d:\n",count++);
		if(n%2==1)
		{
			if(n==1) printf("1\n");
			else printf("\n");
		} 
		else
		{
			dfs(2,n);
			printf("\n");
		}
	}
	return 0;
}

Pay attention to the access arrays and storage arrays to zero.
Furthermore, if an odd number of inputs, it primes will not form a ring, since the prime ring is necessarily an odd number adjacent a thermocouple.
If it is, then 1 direct output 1 on it.