1.5.3 Superprime Rib(dfs)

Butchering Farmer John's cows always yields the best prime rib. You can tell prime ribs by looking at the digits lovingly stamped across them, one by one, by FJ and the USDA. Farmer John ensures that a purchaser of his prime ribs gets really prime ribs because when sliced from the right, the numbers on the ribs continue to stay prime right down to the last rib, e.g.:

     7  3  3  1

The set of ribs denoted by 7331 is prime; the three ribs 733 are prime; the two ribs 73 are prime, and, of course, the last rib, 7, is prime. The number 7331 is called a superprime of length 4.

Write a program that accepts a number N 1 <=N<=8 of ribs and prints all the superprimes of that length.

The number 1 (by itself) is not a prime number.

PROGRAM NAME: sprime

INPUT FORMAT

A single line with the number N.

SAMPLE INPUT (file sprime.in)

4

OUTPUT FORMAT

The superprime ribs of length N, printed in ascending order one per line.

SAMPLE OUTPUT (file sprime.out)

2333

2339

2393

2399

2939

3119

3137

3733

3739

3793

3797

5939

7193

7331

7333

7393

 

代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#define ll long long
using namespace std;
bool p(ll num)
{
    if(num==1||num==0) return false;
    if(num==2) return true;
    for(ll i=2;i*i<=num;i++){
        if(num%i==0) return false;
    }
    return true;
}
void dfs(ll num,ll l)
{
    if(l==0) {cout<<num<<endl;return ;}
    l--;
    for(ll i=0;i<=9;i++){
        if(p(num*10+i))
            dfs(num*10+i,l);
    }
}
int main()
{
    freopen("sprime.in","r",stdin);
    freopen("sprime.out","w",stdout);
    ll len;
    cin>>len;
    dfs(0,len);
    return 0;
}