poj1016 Prime Ring Problem---dfs

Prime Ring Problem

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 22718    Accepted Submission(s): 10116

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

68

Sample Output

Case 1:1 4 3 2 5 61 6 5 2 3 4Case 2:1 2 3 8 5 6 7 41 2 5 8 3 4 7 61 4 7 6 5 8 3 21 6 7 4 3 8 5 2

Source

Asia 1996, Shanghai (Mainland China)

 

 

题目大意：给你一个n，输出用1~n的数组成一个环，相邻两数之间的和为素数。

 

代码：

#include <iostream>#include <cstring>using namespace std;int hash[30];int arr[100];int n;int check(int x){    for(int i=2;i<x;i++)    {        if(x%i==0)        return 0;    }    return 1;}void dfs(int order,int index){    int i;    arr[order]=index;    hash[index]=1;    if(order==n)    {        if(check(arr[order]+arr[1]))        {              cout<<"1";              for(i=2;i<=n;i++)              cout<<" "<<arr[i];              cout<<endl;        }       return;    }    for(i=1;i<=n;i++)    {        if(!hash[i]&&check(arr[order]+i))        {            dfs(order+1,i);            hash[i]=0;        }    }    return;}int main(){    int flag=1;    while(cin>>n)    {        cout<<"Case "<<flag++<<":"<<endl;        memset(hash,0,sizeof(hash));        dfs(1,1);        cout<<endl;    }    return 0;}