POJ 3126 Prime Path（搜索）

AC代码：

#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
#include <queue>
using namespace std;

struct node{
    int pos,t;
    bool operator < (const node &a)const{
        return t > a.t;
    }
};
int prime[10005];
int vis[10005];

void is_prime(){
    memset(prime,0,sizeof(prime));
    for(int i = 1000; i < 10000; i++){
        int tmp = sqrt(i*1.0),flag = 1;
        for(int j = 2; j <= tmp; j++){
            if(i%j == 0){
                flag = 0;
                break;
            }
        }
        if(flag)
            prime[i] = 1;
    }
}

int bfs(int a,int b){
    memset(vis,0,sizeof(vis));
    node cur;
    int num[4];
    cur.pos = a;cur.t = 0;
    vis[a] = 1;
    priority_queue<node> q;
    q.push(cur);
    while(!q.empty()){
        cur = q.top();
        q.pop();
        if(cur.pos == b){
            return cur.t;
        }
        int tmppos = cur.pos;
        for(int i = 3; i >= 0; i--,tmppos/=10){
            num[i] = tmppos%10;
        }
        for(int i = 0; i < 4; i++){
            int tmp = num[i];
            for(int j = 0; j < 10; j++){
                if(num[i] != j){
                    num[i] = j;
                    int pos = num[0]*1000+num[1]*100+num[2]*10+num[3];
                    if(prime[pos] && !vis[pos]){
                        vis[pos] = 1;
                        q.push(node{pos,cur.t+1});
                    }
                }
                num[i] = tmp;
            }
        }
    }
    return -1;
}

int main(){
    is_prime();
    int T;
    scanf("%d",&T);
    while(T--){
        int a,b;
        scanf("%d%d",&a,&b);
        int ans = bfs(a,b);
        if(ans < 0)
            printf("Impossible\n");
        else
            printf("%d\n",ans);
    }
    return 0;
}