hdu 素数环
算法:搜索
题意:相邻的两个数之和是素数,别忘了最后一个,和第一个
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
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
代码:
#include <iostream> #include <string> #include <algorithm> #include <iomanip> #include <cstring> using namespace std; int a[30],b[45],n,m,c[30]; void cmp() { memset(b,1,sizeof(b)); b[1]=b[0]=0; for(int i=2;i<=22;i++) { for(int j=2*i;j<45;j=j+i) b[j]=0; } } void dfs(int x,int k) { if(k==n&&b[c[0]+c[n-1]]) { for(int i=0;i<n;i++) { if(i)cout<<" "; cout<<c[i]; } cout<<endl; } else { for(int i=2;i<=n;i++) { if(!a[i]&&b[i+c[k-1]]) { a[i]=1; c[k]=i; dfs(n,k+1); a[i]=0; } } } } int main() { int i,j=0; cmp(); while(cin>>n) { for(i=1;i<=n;i++) a[i]=0; c[0]=1; cout<<"Case "<<++j<<":"<<endl; dfs(n,1); cout<<endl; } return 0; }