Prime Path (UVA 12101, SPOJ PPATH, POJ 3126, HDU 1973, UVALive 3639, Regionals 2006 >> Europe - Northwestern)

///     Raihan Ruhin
///     CSE, Jahangirnagar University.
///     Dhaka-Bangladesh.
///     id: raihanruhin (topcoder / codeforces / codechef / uva / uvalive / spoj), 3235 (lightoj)
///     mail: raihanruhin@ (yahoo / gmail / facebook)
///     blog: ruhinraihan.blogspot.com

#include<bits/stdc++.h>
using namespace std;

#define SET(a) memset(a,-1,sizeof(a))
#define CLR(a) memset(a,0,sizeof(a))
#define PI acos(-1.0)

#define MOD 1000000007
#define MX 10000

vector<int>prime;
bool status[MX+2];
int pl;

void PrimeGenerate(int n)
{
    int sq=sqrt(n);
    for(int i=3; i<=sq; i+=2)
    {
        if(!status[i])
            for(int j=i*i; j<=n; j+=(i<<1))
                status[j]=true;
    }

    status[0]=status[1]=true;
    for(int i=4;i<=n;i+=2) status[i]=true;

    prime.push_back(2);
    for(int i=3; i<=n; i+=2)
        if(!status[i])
            prime.push_back(i);
return;
}

int dp[MX], st, ed, taken[MX], dist[10010];
vector<int>v[10010];

int bfs(int num)
{
    if(num==ed) return 1;
    dist[num]=1;
    queue<int>Q;
    Q.push(num);
    while(!Q.empty())
    {
        int qq=Q.front();
        Q.pop();
        for(int i=0;i<v[qq].size();i++)
            if(!dist[v[qq][i]])
            {
                dist[v[qq][i]]=dist[qq]+1;
                if(v[qq][i]==ed) return dist[v[qq][i]];
                Q.push(v[qq][i]);
            }
    }
return 100000;
}

int main()
{
    ios_base::sync_with_stdio(0);cin.tie(0);
    int tc, kk=1, n;
    PrimeGenerate(MX);
    int loopst;

    pl=prime.size();
    for(int i=0;i<pl;i++)
        if(prime[i]>999)
        {
            loopst=i;
            break;
        }

    for(int i=loopst;i<pl;i++)
    {

        for(int j=loopst;j<pl;j++)
        {
            int tmp=prime[i];
            int tmp2=prime[j], cnt=0;

            while(tmp && tmp2)
            {
                if(tmp%10 != tmp2%10) cnt++;
                tmp/=10;
                tmp2/=10;
            }

            if(cnt==1)
            {
                v[prime[i]].push_back(prime[j]);
            }
        }
    }

    cin>>tc;
    while(tc--)
    {
        cin>>st>>ed;
        if(ed>st) swap(st, ed);
        CLR(dist);
        int ans=bfs(st);
        if(ans<100000) cout<<ans-1<<"\n";
        else cout<<"Impossible\n";
    }

return 0;
}