uva 524 prime ring problem——yhx

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Prime Ring Problem

A ring is composed of n (even number) circles as shown in diagram. Put natural numbers $1, 2, \dots, n$ 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 <= 16)

Output

The output format is shown as sample below. Each row represents a seriesof circle numbers in thering beginning from 1 clockwisely and anticlockwisely. The order of numbersmust satisfy the above requirements.

You are to write a program that completes above process.

 1 #include<cstdio> 2 #include<cstring> 3 bool prm[35],vis[20]; 4 int a[20],n; 5 bool ck(int x) 6 { 7     int i; 8     for (i=2;i*i<=x;i++) 9       if (x%i==0) return 0;10     return 1;11 }12 void dfs(int p)13 {14     int i,j,k,x,y,z;15     if (p==n+1)16     {17         if (prm[a[n]+a[1]])18         {19             printf("%d",a[1]);20             for (i=2;i<=n;i++)21               printf(" %d",a[i]);22             printf("\n");23         }24         return;25     }26     for (i=2;i<=n;i++)27       if (vis[i]==0&&prm[i+a[p-1]])28       {29           a[p]=i;30           vis[i]=1;31           dfs(p+1);32           vis[i]=0;33       }34 }35 int main()36 {37     int i,j,k,p,q,x,y,z,t;38     bool bbb=0;39     for (i=2;i<=35;i++)40       prm[i]=ck(i);41     a[1]=1;42     t=0;43     while (scanf("%d",&n)==1)44     {45         if (bbb) printf("\n");46         bbb=1;47         memset(vis,0,sizeof(vis));48         vis[1]=1;49         printf("Case %d:\n",++t);50         dfs(2);51     }52 }

素数环。注意边界。注意每组数据间的回车（虽然题上没说）。

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