九度 Prime Ring Problem hdu 1016

题目来源：http://acm.hdu.edu.cn/showproblem.php?pid=1016

题目来源：http://ac.jobdu.com/problem.php?pid=1459

时间限制：2 秒

内存限制：128 兆

特殊判题：否

提交：803

解决：330

题目描述：

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.

输入：

n (1 < n < 17).

输出：

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.

样例输入：

68

样例输出：

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

提示：

用printf打印输出。

回溯搜索

#include <iostream>#include <cstdio>#include <cstring>using namespace std;const int MAXN = 50;int IsPrime[MAXN], visit[MAXN>>1];void PreCheckPrime(){    int i, j;    for(i = 0; i < MAXN; ++i)        IsPrime[i] = 1;    IsPrime[0] = IsPrime[1] = 0;    for(i = 2; i < MAXN; ++i)    {        if(IsPrime[i])        {            for(j = i+i; j < MAXN; j += i)                IsPrime[j] = 0;        }    }}void Print_Prime_Ring(int* arr, int n){    for(int i = 1; i < n; ++i)        printf("%d ", arr[i]);    printf("%d\n", arr[n]);}void FindPrimeRing(int n, int index, int* arr){    if(index == n+1)    {        if(IsPrime[arr[index-1] + 1])            Print_Prime_Ring(arr, n);        return ;    }    for(int i = 2; i <= n; ++i)    {        if(!visit[i] && IsPrime[i+arr[index-1]])        {            visit[i] = 1;            arr[index] = i;            FindPrimeRing(n, index+1, arr);            visit[i] = 0;        }    }}int main(){    int n, kcase = 1;    PreCheckPrime();    while(~scanf("%d", &n))    {        printf("Case %d:\n", kcase++);        int arr[MAXN];        memset(visit, 0, sizeof(visit));        arr[1] = 1;        visit[1] = 1;        FindPrimeRing(n, 2, arr);        printf("\n");    }    return 0;}