— It is a matter of security to change such things every now and then, to keep the enemy in the dark.
— But look, I have chosen my number 1033 for good reasons. I am the Prime minister, you know!
— I know, so therefore your new number 8179 is also a prime. You will just have to paste four new digits over the four old ones on your office door.
— No, it’s not that simple. Suppose that I change the first digit to an 8, then the number will read 8033 which is not a prime!
— I see, being the prime minister you cannot stand having a non-prime number on your door even for a few seconds.
— Correct! So I must invent a scheme for going from 1033 to 8179 by a path of prime numbers where only one digit is changed from one prime to the next prime.
Now, the minister of finance, who had been eavesdropping, intervened.
— No unnecessary expenditure, please! I happen to know that the price of a digit is one pound.
— Hmm, in that case I need a computer program to minimize the cost. You don't know some very cheap software gurus, do you?
— In fact, I do. You see, there is this programming contest going on... Help the prime minister to find the cheapest prime path between any two given four-digit primes! The first digit must be nonzero, of course. Here is a solution in the case above.
1033The cost of this solution is 6 pounds. Note that the digit 1 which got pasted over in step 2 can not be reused in the last step – a new 1 must be purchased.
1733
3733
3739
3779
8779
8179
3 1033 8179 1373 8017 1033 1033Sample Output
6 7 0
解題思路:
題意就是將一個數轉換成另一個數,轉換的過程中還要保證是素數,這個也是bfs能解決的,分別對於個位十位百位千位都處理,要注意其實偶數一定不是素數的,所以對於個位可以用2的幅度去加,而千位是沒有0的,而對於素數的判斷,除了小於sqrt()之外,還有素數篩,比如2的倍數都不是素數,那麼就把2的倍數都篩掉,這樣一直下去,會很快
代碼:
#include<iostream>
#include<queue>
#include<cstring>
#include<cmath>
using namespace std;
int beginn,endd;
int vis[10005];
struct node
{
int x;
int step;
};
bool ok(int a)
{
if(a==2||a==3)
return true;
if(a==1||a==0)
return false;
for(int i = 2;i<=(int)sqrt(a);i++)
{
if(a%i==0)
return false;
}
return true;
}
int bfs(int now)
{
node help;
node help1;
help.x = now;
help.step = 0;
queue<node> q;
q.push(help);
vis[now] = 1;
while(!q.empty())
{
// cout<<"here"<<endl;
help = q.front();
q.pop();
if(help.x==endd)
{
// cout<<"here"<<endl;
return help.step;
}
for(int i = 1;i<=9;i+=2)
{
int s = help.x/10*10+i;
// cout<<"gewei "<<s<<endl;
help1.x = s;
help1.step = help.step+1;
if(!vis[help1.x]&&ok(help1.x))
{
q.push(help1);
vis[help1.x] = 1;
}
}
for(int i = 0;i<=9;i++)
{
int s = help.x/100*100+i*10+help.x%10;
//cout<<"baiwei "<<s<<endl;
help1.x = s;
help1.step = help.step+1;
if(!vis[help1.x]&&ok(help1.x))
{
q.push(help1);
vis[help1.x] = 1;
}
}
for(int i = 0;i<=9;i++)
{
int s = help.x/1000*1000+i*100+help.x%100;
//cout<<"qianwei "<<s<<endl;
help1.x = s;
help1.step = help.step+1;
if(!vis[help1.x]&&ok(help1.x))
{
q.push(help1);
vis[help1.x] = 1;
}
}
for(int i = 1;i<=9;i++)
{
int s = i*1000+help.x%1000;
//
help1.x = s;
help1.step = help.step+1;
if(!vis[help1.x]&&ok(help1.x))
{
// cout<<"hisudfh"<<endl;
q.push(help1);
vis[help1.x] = 1;
}
}
}
return -1;
}
int main()
{
int t;
cin>>t;
while(t--)
{
memset(vis,0,sizeof(vis));
cin>>beginn>>endd;
int re = bfs(beginn);
cout<<re<<endl;
}
}