hdu1016 Prime Ring problem (DFS)

This topic is the primary topic of DFS, but I am almost ignorant of Dfs, so it is very difficult to write this question, but write this after the other topic is very easy, the problem is to find prime ring, a very basic deep search

#include <iostream> #include <string.h> using namespace std;
int prime[41]={0,0,1,1,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,0,0,0,0,1,0,1,0,0,0,0,0,1,0,0,0};
int n;
BOOL VIS[25];
int a[25];
    void Dfs (int pos)//This is where the location {//cout<<pos<<endl;
            if (Pos==n+1&&prime[a[n]+a[1]])//This must be n+1, because at n time, the nth bit has not processed {for (int i=1;i<pos-1;i++) {
        cout<<a[i]<< "";
        } cout<<a[pos-1]<<endl;
        cout<<endl;
    Return
                } for (int i=2;i<=n;i++) {if (!vis[i]&&prime[i+a[pos-1]]) {
                A[pos]=i;
                Vis[i]=true;
                DFS (POS+1);
            Vis[i]=false;
    }}} int main () {int cases=1;
        while (cin>>n) {cout<< ' case ' <<cases++<< ': ' <<endl;
        memset (vis,0,sizeof (VIS));
        A[1]=1;
        Vis[1]=true;
        DFS (2);cout<<endl;
} return 0;
 }