[HDOJ1016]Prime Ring Problem

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1016

原题：

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个数字，求1到N这N个数字连成环以后两两相加均为素数的所有情况。

非常经典的DFS题。

#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
using namespace std;
int n;
int cir[];
int prime[];
int vis[];
int isPrime(int x)
{
    int i;
    int sqx = sqrt((float)x);
    for(i = ; i <= sqx; i++)
    {
        if(x % i == )
        {    
            return ;
        }
    }
    return ;
}

void dfs(int x)
{
    int i;
    if(x == n && prime[cir[]+cir[n]])    //make a circle
    {
        for(i = ; i < n; i++)
        {
            printf("%d ", cir[i]);
        }
        printf("%d\n", cir[n]);
        return ;
    }
    for(i = ; i <= n; i++)
    {
        if(vis[i] ==  && prime[cir[x]+i])
        {
            cir[x+] = i;
            vis[i] = ;
            dfs(x+);
            vis[i] = ;    //reset
        }
    }
}

int main()
{
    int i, Case = ;
    for(i = ; i < ; i++)    //list primes.
    {
        if(isPrime(i))
        {
            prime[i] = ;
        }
    }

    while(scanf("%d", &n) != EOF && n)
    {
        printf("Case %d:\n", Case++);
        memset(vis, , sizeof(vis));
        cir[] = ;
        vis[] = ;
        dfs();
        printf("\n");
    }
    return ;
}