BFS/poj 3126 prime path

#include<cstdio>
#include<queue>
#include<cstring>
#include<cmath>
using namespace std;
int n,x,y;
bool v[20000];
struct point
{
    int x;
    int step;
};

int change(int x,int y,int z)
{
    if (z==4) return (x/10)*10+y;
    else if (z==3)
    {
        int t=y*10+x%10;
        return (x/100)*100+t;
    }
    else if (z==2)
    {
        int t=y*100+x%100;
        return (x/1000)*1000+t;
    }
    else if (z==1)
    {
        return y*1000+(x%1000);
    }
}

bool prime(int x)
{
    if (x==2 || x==3) return true;
    if (x<=1 || x%2==0) return false;
    for (int i=3;i<=sqrt(x);i++)
        if (x%i==0) return false;
    return true;
}

void bfs(int sx,int ex)
{
    memset(v,0,sizeof(v));
    queue<point>que;
    point now,next;
    now.x=sx;
    now.step=0;
    v[sx]=1;
    que.push(now);
    while (!que.empty())
    {
        now=que.front();
        que.pop();
        if (now.x==ex)
        {
            printf("%d\n",now.step);
            return ;
        }

        for (int i=0;i<=9;i++)
            for (int j=1;j<=4;j++)
            {
                if (i==0 && j==1) continue;
                next.x=change(now.x,i,j);
                if (prime(next.x) && !v[next.x])
                {
                    v[next.x]=1;
                    next.step=now.step+1;
                    que.push(next);
                }
            }

    }
    printf("Impossible\n");
}
int main()
{
    scanf("%d",&n);
    for (int t=1;t<=n;t++)
    {
        scanf("%d%d",&x,&y);
        bfs(x,y);
    }
    return 0;
}