HDU 1016--Prime Ring Problem【DFS】

Prime Ring Problem

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 30934    Accepted Submission(s): 13725

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

 

以前做过的题，再做一遍也蛮有收获的。

#include <cstdio>#include <cstring>#include <cmath>int n;int a[25];int vis[25];bool check(int n){int i;for(i=2;i<=sqrt((double)n);++i)if(n%i==0)return 0;if(n==0 || n==1)return 0;return 1;}void dfs(int pos){    int i;if(pos>n){if(check(a[n]+a[1])){for(i=1;i<n;++i)printf("%d ",a[i]);printf("%d\n",a[n]);}}for(i=1;i<=n;++i){if(check(i+a[pos-1]) && !vis[i]){a[pos]=i;vis[i]=1;dfs(pos+1);vis[i]=0;}}}int main (){    int ans=1;while(scanf("%d",&n)!=EOF){memset(vis,0,sizeof(vis));printf("Case %d:\n",ans++);a[1]=1;vis[1]=1;dfs(2);printf("\n");}return 0;}