UV 552 prime ring Problem

Link to the question: Ultraviolet A 552

Ideas:

The time limit is 3 s, and the data is small. Use deep search to find all solutions.

 

Code:

#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;}

 

UV 552 prime ring Problem