题意：给一个四位数的素数，求通过几步变换（一次只能换一个位置的数，且变换过程中只能出现素数）变为目标四位数

分析：素数筛法+BFS，BFS时更换一个数字如果符合条件就加入队列

代码：

#include <map>
#include <set>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <vector>
#include <iomanip>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iostream>
#include <algorithm>
#define ll long long
#define mod 1000000007
#define mem(a) memset(a,0,sizeof(a))using namespace std;typedef pair <int,int> pii;
const int maxn = 10000 + 5 , inf = 0x3f3f3f3f;
bool vis[maxn];
void isPrime(){memset(vis,true,sizeof(vis));vis[1]=false;vis[2]=true;for(int i=2;i<=maxn;i++){for(int j=i*2;j<=maxn;j+=i){vis[j]=false;}}
}
struct node{int x,d;node(int xx,int dd):x(xx),d(dd){};
};
bool done[maxn];
int Start,End;
int bfs(int d){mem(done);queue<node>q;while(!q.empty())q.pop();q.push(node(Start,d));while(!q.empty()){node now = q.front();q.pop();if(now.x==End) return now.d;else{for(int i=1;i<=9;i+=2){int temp = now.x/10*10+i;if(vis[temp]&&temp!=now.x&&!done[temp]){done[temp]=true;q.push(node(temp,now.d+1));}}for(int i=0;i<=9;i++){int nn = now.x%10;int temp = now.x/100*100+i*10+nn;if(vis[temp]&&temp!=now.x&&!done[temp]){done[temp]=true;q.push(node(temp,now.d+1));}}for(int i=0;i<=9;i++){int nn = now.x%100;int temp = now.x/1000*1000+i*100+nn;if(vis[temp]&&temp!=now.x&&!done[temp]){done[temp]=true;q.push(node(temp,now.d+1));}}for(int i=1;i<=9;i++){int nn = now.x%1000;int temp = i*1000+nn;if(vis[temp]&&temp!=now.x&&!done[temp]){done[temp]=true;q.push(node(temp,now.d+1));}}}}return -1;
}int main(){//freopen("in.txt","r",stdin);//freopen("out.txt","w",stdout);int T;scanf("%d",&T);isPrime();while(T--){cin>>Start>>End;int ans = bfs(0);if(ans==-1) cout<<"Impossible"<<endl;else cout<<ans<<endl;}
}