【Poj3126】【BNUOJ3245】Prime Path

http://poj.org/problem?id=3126

https://www.bnuoj.com/v3/problem_show.php?pid=3245

题目鬼图

刚开始看到题目的图觉得这题会很鬼,然后看题,发现是水题。。。

线性筛预处理素数→BFS+记录深度 or 迭代加深 or 分层BFS

// <3126.cpp> - 11/01/16 17:31:53
// This file is made by YJinpeng，created by XuYike's black technology automatically.
// Copyright (C) 2016 ChangJun High School, Inc.
// I don't know what this program is.

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <queue>
using namespace std;
typedef long long LL;
const int MAXN=10010;
inline int gi() {
	register int w=0,q=0;register char ch=getchar();
	while((ch<'0'||ch>'9')&&ch!='-')ch=getchar();
	if(ch=='-')q=1,ch=getchar();
	while(ch>='0'&&ch<='9')w=w*10+ch-'0',ch=getchar();
	return q?-w:w;
}
queue<int>q;int f[MAXN],t[11];bool prime[MAXN];
int bfs(int u,int v){
    if(u==v)return 0;
    while(!q.empty())q.pop();
    q.push(u);
    memset(f,0,sizeof(f));
    while(!q.empty()){
        int x=q.front();q.pop();
        t[0]=x/1000;t[1]=x%1000/100;t[2]=x%100/10;t[3]=x%10;
        for(int i=0;i<4;i++){
            int g=t[i];
            for(int j=i==0?1:0;j<=9;j++)
                if(j!=g){
                    t[i]=j;
                    int k=t[0]*1000+t[1]*100+t[2]*10+t[3];
                    if(!f[k]&&!prime[k]){
                        f[k]=f[x]+1;q.push(k);
                        if(k==v)return f[k];
                    }
                }
            t[i]=g;
        }
    }
    return -1;
}
int main()
{
	freopen("3126.in","r",stdin);
	freopen("3126.out","w",stdout);
    prime[1]=1;
    for(int i=2;i<MAXN;i++)
        if(!prime[i])
            for(int j=i+i;j<MAXN;j+=i)prime[j]=1;
    int t=gi();
    while(t--){
        int u=gi(),v=gi(),ans=bfs(u,v);
        if(ans==-1)printf("Impossible\n");else printf("%d\n",ans);
    }
	return 0;
}