hdu 1016 Prime Ring Problem
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Prime Ring Problem
Time Limit: 4000/2000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)Total Submission(s): 51048 Accepted Submission(s): 22596
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.
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.
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
题目大意:给你一个数字n,输出素数环(即环中任何两个数字之和是素数)。
题目解析:
这是一个简单的模拟题;
素数打表+DFS
代码:
#include<cstdio>#include<cstring>using namespace std;//判断是否为素数,是返回1,不是返回0int a[20],vis[20],isprime[45]={0},n;void get_prime(){int i,j;for(i=2;i<8;i++) if(!isprime[i]) for(j=i*i;j<45;j+=i) isprime[j]=1;}int dfs(int step){ int i; if(step==n+1&&!isprime[a[n]+a[1]])//搜索结束条件 { for(i=1;i<n;i++) printf("%d ",a[i]); printf("%d\n",a[n]); return 0; } for(i=2;i<=n;i++) { if(!vis[i]&&!isprime[i+a[step-1]]) { a[step]=i; vis[i]=1; dfs(step+1); vis[i]=0; } }}int main(){ int cas=1; a[1]=1; get_prime(); while(~scanf("%d",&n)) { memset(vis,0,sizeof(vis)); printf("Case %d:\n",cas++); dfs(2); printf("\n"); } return 0;}
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