D - Prime Ring Problem HDU - 1016 DFS水题

D - Prime Ring Problem HDU - 1016 

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

素数环问题：在环中填入1~n，每个数和相邻的数之和均为素数，输出所有的方案（以1开头）

 暴力DFS，第一个位置填1，然后一个位置一个位置地填1~n里没被填过的数，判断和上一个位置的和是否为素数，到最后一个要记得判断+1是否是素数

#include <iostream>
#include <cstring>
#include <cstdio>
#include <queue>
#include <cmath>
#include <algorithm>
#include <string>
#include <map>
#include <stack>
#include <set>

using namespace std;
const int maxn=33;
int ans[maxn];
int f[maxn];
int n;
bool ss(int x)
{
    for(int i=2;i<=sqrt(x);i++)
    {
        if(x%i==0)return false;
    }
    return true;
}
bool OK(int i,int x)
{
    if(f[x]||!ss(x+ans[i-1]))return false;
    return true;
}
void print()
{
    for(int i=1;i<=n;i++)
    {
        if(i!=1)printf(" ");
        printf("%d",ans[i]);
    }
    printf("\n");
}
void DFS(int x,int n)
{
    if(x==n+1)
    {
        if(ss(ans[x-1]+ans[1]))print();
        return;
    }
    for(int i=2;i<=n;i++)
    {
        if(OK(x,i))
        {
            f[i]=1;
            ans[x]=i;
            DFS(x+1,n);
            f[i]=0;
        }

    }
}
int main()
{
    int T=0;
    while(~scanf("%d",&n))
    {
        printf("Case %d:\n",++T);
        ans[1]=1;
        memset(f,0,sizeof(f));
        f[1]=1;
        DFS(2,n); printf("\n");
    }
    return 0;
}