Backtracking: Prime Rings

#include<iostream>
using namespace std;
int n = 8;
int a[20];// Storing Prime Rings 
bool vis[21];
bool isPrime(int x) {
    
	for (int i = 2; i <= x / i; i++) {
    
		if (x % i == 0) return false;
	}
	return true;
}
void dfs(int beg) {
    
	if (beg == n && isPrime(a[0] + a[n - 1])){
    
		for (int i = 0; i < n; i++) 
			cout << a[i]<<" ";
		cout << endl;
		return;
	}

	for (int j = 1; j <= n; j++)
		if (!vis[j] && isPrime(j + a[beg - 1])) {
    
			a[beg] = j;
			vis[j] = true;
			dfs(beg + 1);
			vis[j] = false;
		}
}
int main()
{
    
	a[0] = 1;//1 Set the position , It's a ring. , All the first elements do not affect the overall trend 
	vis[1] = true;
	dfs(1);
}