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## 。。。

Posted by 920394589 at 2017-08-17 22:52:21 on Problem 3126
```#include<stdio.h>
#include<string.h>
#include<queue>
#include<vector>
using namespace std;

const int maxn =10000;
int tb[maxn];
int d[maxn];
vector<int> p,arr[maxn];

void getTable(){
for(int i=2;i*i<maxn;++i){
if(!tb[i]){
for(int j=i*2;j<maxn;j+=i){
tb[j] = 1;
}
}
}
for(int i=1000;i<10000;++i){
if(!tb[i])p.push_back(i);
}
}

int isVaild(int x,int y){
int diff=0;
for(int i=0;i<4;++i){
diff+=(x%10!=y%10);
x/=10;y/=10;
}
return diff==1;
}

void getVaild(){
for(int i=0;i<p.size();++i){
for(int j=0;j<p.size();++j){
if(isVaild(p[i],p[j])){
arr[p[i]].push_back(p[j]);
}
}
}
}

int bfs(int a,int b){
if(a==b)return 0;
queue<int> q;
q.push(a);
d[a]=0;
while(q.size()){
int x = q.front();
q.pop();
for(int i=0;i<arr[x].size();++i){
int y = arr[x][i];
if(y==b)return d[x]+1;
if(d[y]==-1){
d[y]=d[x]+1;
q.push(y);
}
}
}
return -1;
}

int main(){
getTable();
getVaild();
int t;
scanf("%d",&t);
while(t--){
int a,b;
scanf("%d%d",&a,&b);
memset(d,-1,sizeof(d));
int ret = bfs(a,b);
if(ret==-1)printf("Impossible\n");
else printf("%d\n",ret);
}
return 0;
}```

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