Uva 552 Prime Ring Problem

#include <iostream>
#include <string.h>
using namespace std;

const int MAX_N = 20;
int n;
int A[MAX_N], vis[MAX_N];

int is_prime( int n )
{
    if ( n == 2 )
        return true;

    for ( int i = 2; i * i <= n; ++i )
    if ( n % i == 0 )
        return false;

    return true;
}

void dfs( int cur )
{
    if ( cur == n && is_prime( A[0] + A[n - 1] ) )
    {
        int i = 0;

        for ( i = 0; i < n - 1; ++i )
            printf( "%d ", A[i] );
        printf( "%d", A[i] );
        printf( "\n" );
    }
    else
    {
        for ( int i = 2; i <= n; ++i )
        {
            if ( !vis[i] && is_prime( i + A[cur - 1] ) )
            {
                A[cur] = i;
                vis[i] = 1;
                dfs( cur + 1 );
                vis[i] = 0;
            }
        }
    }
}

int main( )
{
    int count = 1;

    while ( scanf( "%d", &n ) != EOF )
    {
        if ( count != 1 )
            printf( "\n" );

        A[0] = 1;
        memset( vis, 0, sizeof( vis ) );
        printf( "Case %d:\n", count++ );
        dfs( 1 );
    }

    return 0;
}