POJ 1475 Pushing Boxes 嵌套BFS -

题目地址：http://poj.org/problem?id=1475

题意Push路程要最少，所以直接找box到T的最短路径

box能动的前提是有个man能直接在他后面推他

所以再加一个BFS找man到他后面的最短路径

然后思考要优选队列里要保存哪些信息

box在何处 这个肯定要

还要保存路径啊，题目求的就是这个，所以box从起点到该点路径要保存，那么直接放个Path字符串保存一下路径就好了

然后要求man到box屁股后面的路径，那必须要知道man原来在哪里对不对，所以再保存个man点

算法就是BFS box走一步时再BFS man看看能不能走

AC代码如下：

#include<cstdio>#include<cstring>#include<queue> #include<string> #include<algorithm>#include<iostream>using namespace std;const int maxn=20+5;int R,C;char G[maxn][maxn];struct Point{int x,y;Point(int x=0,int y=0):x(x),y(y){} bool operator == (const Point& p){return x==p.x&&y==p.y;}bool operator != (const Point& p){return !(*this==p);}}box,terminal,start;struct Node{Point box;Point man;string path; //man保存此box时man的位置 Node(Point b,Point man,string p):box(b),man(man),path(p){}Node(Point man,string p):man(man),path(p){}};const int dx[]={-1,1,0,0};const int dy[]={0,0,-1,1};const char d[]="NSWEnswe";void Print(Point p){cout<<p.x<<' '<<p.y<<endl;}Point move(Point p,int i){return Point(p.x+dx[i],p.y+dy[i]);}bool inside(Point p){return p.x>=1&&p.x<=R&&p.y>=1&&p.y<=C;}string BFS_MAN(Point s,Point t,Point box) //s->t 中间不能经过box {queue<Node> Q;bool vis[maxn][maxn]={false};Q.push(Node(s,""));while(!Q.empty()){Point u=Q.front().man; string path=Q.front().path; Q.pop();if(u==t) return path;for(int i=0;i<4;i++){Point v=move(u,i);if(!inside(v)||G[v.x][v.y]=='#'||box==v||vis[v.x][v.y]) continue;vis[v.x][v.y]=true;Q.push(Node(v,path+d[i+4]));}}return "";}bool BFS_BOX(){queue<Node> Q;bool vis[maxn][maxn][4]={false};Q.push(Node(box,start,"")); while(!Q.empty()){Point ubox=Q.front().box,uman=Q.front().man; string path=Q.front().path; Q.pop();if(ubox==terminal) {cout<<path<<endl;return true;}for(int i=0;i<4;i++){Point vbox=move(ubox,i);if(!inside(vbox)||G[vbox.x][vbox.y]=='#'||vis[vbox.x][vbox.y][i]) continue;Point vman=move(ubox,i^1);  //反方向走 if(!inside(vman)||G[vman.x][vman.y]=='#') continue;string ManPath;if(vman!=uman){ManPath=BFS_MAN(uman,vman,ubox);if(ManPath.empty()) continue;}vis[vbox.x][vbox.y][i]=true;Q.push(Node(vbox,ubox,path+ManPath+d[i]));}}return false;}int main(){int kase=0;while(cin>>R>>C){if(R==0&&C==0) break;for(int i=1;i<=R;i++) {cin>>(G[i]+1);for(int j=1;j<=C;j++)if(G[i][j]=='T') terminal=Point(i,j);else if(G[i][j]=='B') box=Point(i,j);else if(G[i][j]=='S') start=Point(i,j);}printf("Maze #%d\n",++kase);if(!BFS_BOX()) cout<<"Impossible."<<endl; cout<<endl; }return 0;}