HDU 1242 (BFS search + priority queue)

Question Link: Http://acm.hdu.edu.cn/showproblem.php? PID = 1, 1242

question .

solution :

If there is no special point, it is common BFs.

Because of the intervention of special points, a point may arrive in different ways, so the priority queue is used. For a point, the method used to retrieve the first point must be optimal.

At the same time, the use of priority queue also provides a pruning for the BFS process, because the first arrival is certainly the best result. As long as the first result is found, no search is needed.

Note that there are multiple start points, so first record all the start points and find the minimum BFS in sequence.

 

#include "cstdio"#include "string"#include "cstring"#include "iostream"#include "vector"#include "queue"using namespace std;#define inf 1<<28struct status{    int x,y,dep;    status(int x,int y,int dep):x(x),y(y),dep(dep) {}    bool operator < (const status &a) const    {        return dep>a.dep;    }};int n,m,map[205][205],vis[205][205],dir[4][2]={-1,0,1,0,0,-1,0,1},ans;void bfs(int x,int y){    priority_queue<status> Q;    Q.push(status(x,y,0));    bool flag=false;    while(!Q.empty())    {        if(flag) break;        status t=Q.top();Q.pop();        vis[t.x][t.y]=true;        for(int s=0;s<4;s++)        {            int X=t.x+dir[s][0],Y=t.y+dir[s][1];            if(vis[X][Y]||X<0||X>n||Y<0||Y>n||!map[X][Y]) continue;            if(map[X][Y]==2) {flag=true;ans=min(ans,t.dep+1);};            if(map[X][Y]==3) Q.push(status(X,Y,t.dep+2));            else Q.push(status(X,Y,t.dep+1));        }    }}int main(){    //freopen("in.txt","r",stdin);    ios::sync_with_stdio(false);    string tt;    while(cin>>n>>m&&n)    {        vector<status> f;        ans=inf;        for(int i=1;i<=n;i++)        {            cin>>tt;            for(int j=0;j<tt.size();j++)            {                if(tt[j]==‘#‘) map[i][j+1]=0;                if(tt[j]==‘.‘) map[i][j+1]=1;                if(tt[j]==‘a‘) map[i][j+1]=2;                if(tt[j]==‘x‘) map[i][j+1]=3;                if(tt[j]==‘r‘) {map[i][j+1]=4;f.push_back(status(i,j+1,0));}            }        }        for(int i=0;i<f.size();i++)        {            memset(vis,0,sizeof(vis));            bfs(f[i].x,f[i].y);        }        if(ans!=inf) cout<<ans<<endl;        else cout<<"Poor ANGEL has to stay in the prison all his life."<<endl;    }}

 

11867790

00:53:01

Accepted

1242

46 Ms

644 K

1840 B

C ++

Physcal

 

 

 

HDU 1242 (BFS search + priority queue)