POJ 3126 Prime Path

#include<cstdio>
#include<iostream>
#include<string>
#include<cstring>
#include<queue>
#include<vector>
#include<stack>
#include<vector>
#include<map>
#include<set>
#include<algorithm>
#include<cmath>
//#include<bits/stdc++.h>
using namespace std;

typedef long long ll;

const int maxn = 1e5;
bool isNot[maxn];

void seive(int n = maxn-1)
{
    int m = sqrt(n+0.5);
    for(int i = 2; i <= m; i++){
        if(!isNot[i])
        for(int j = i*i; j <= n; j += i){
            isNot[j] = true;
        }
    }
    isNot[0] = isNot[1] = true;
}

int d[maxn];
int vis[2][maxn],clk;
#define PB push_back
#define PS push
const int wei[] = {1,10,100,1000};

void doubleBfs(int s,int t)
{
    if(s == t) { puts("0"); return; }
    queue<int> Q[3];
    queue<int> *q1 = Q, *q2 = Q+1, *nxt = Q+2;
    int *v1 = *vis, *v2 = vis[1];
    q1->PS(s); q2->PS(t);
    v1[s] = ++clk; v2[t] = clk;
    d[s] = 0; d[t] = 0;
    while(q1->size() && q2->size()){
        if(q1->size()>q2->size()){
            swap(q1,q2); swap(v1,v2);
        }
        while(q1->size()){
            int u = q1->front(); q1->pop();
            int dig[4];
            for(int i = 0; i < 4; i++){
                dig[i] = u/wei[i]%10;
            }
            for(int i = 0; i < 4; i++){
                int t = u - dig[i]*wei[i];
                for(int j = i == 3; j < 10; j++){
                    if(j != dig[i]){
                        int v = t+j*wei[i];
                        if(!isNot[v] && v1[v] != clk){
                            if(v2[v] == clk) {
                                printf("%d\n",d[v]+d[u]+1);
                                return;
                            }
                            d[v] = d[u]+1;
                            nxt->PS(v);
                            v1[v] = clk;
                        }
                    }
                }
            }
        }
        swap(q1,nxt);
    }
    puts("Impossible");
}

//#define LOCAL
int main()
{
#ifdef LOCAL
    freopen("in.txt","r",stdin);
#endif
    seive();
    int T; scanf("%d",&T);
    while(T--){
        int a,b; scanf("%d%d",&a,&b);
        doubleBfs(a,b);
    }
    return 0;
}