poj 1475 Pushing Boxes

双重bfs：

　　在推箱子游戏中人推箱子只有两种情况：

　　　　人推箱子：那么箱子此时所处的位置就是人接下来会到达的位置

　　　　人找位子推箱子：那么人就要到达箱子下一步到达位子对应的相反方向得那一个格子

　　所以，主干是箱子的移动过程，箱子每移动一步就对接下来人所处的位子进行考虑。也就是在一次bfs的过程中不断套用另一个bfs

#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <queue>
using namespace std;
const int maxn = 22;
char map[maxn][maxn];
bool visPerson[maxn][maxn];
bool visBox[maxn][maxn];
int R , C;
int dir[4][2] = { {0,1},{0,-1},{1,0},{-1,0} };
char pushes[4] = { 'E','W','S','N' };
char walks[4] = { 'e','w','s','n' };
string path;
struct Node{
    int br , bc , pr , pc;
    string ans;
};
bool inMap(int r , int c) {
    return r>=0 && r<R && c>=0 && c<C;
}
bool bfs2(int sr, int sc, int er, int ec, int br, int bc, string &ans) {
    Node node , tmpnode;
    memset(visPerson,0,sizeof(visPerson));
    queue<Node> Q;
    node.pr = sr;
    node.pc = sc;
    node.ans = "";
    Q.push(node);
    visPerson[br][bc] = true;
    while(!Q.empty()) {
        node = Q.front();  Q.pop();
        if(node.pr == er && node.pc == ec) {
            ans = node.ans;
            return true;
        }
        if(visPerson[node.pr][node.pc]) continue;
        visPerson[node.pr][node.pc] = true;
        for(int i=0;i<4;i++) {
            int nr = node.pr + dir[i][0];
            int nc = node.pc + dir[i][1];
            if(inMap(nr,nc) && !visPerson[nr][nc] && map[nr][nc]!='#') {
                tmpnode.pr = nr;
                tmpnode.pc = nc;
                tmpnode.ans =node.ans + walks[i];
                Q.push(tmpnode);
            }
        }
    }
    return false;
}
bool bfs1(int sr ,int sc, int br, int bc) {
    Node node , tmpnode;
    memset(visBox,0,sizeof(visBox));
    queue<Node> Q;
    node.pr = sr;
    node.pc = sc;
    node.br = br;
    node.bc = bc;
    node.ans = "";
    Q.push(node);
    while(!Q.empty()) {
        node = Q.front();  Q.pop();
        if(visBox[node.br][node.bc])
            continue;
        visBox[node.br][node.bc] = true;
        if(map[node.br][node.bc]=='T') {
            path = node.ans;
            return true;
        }
        visBox[node.br][node.bc] = true;
        for(int i=0;i<4;i++) {
            int nextr = node.br + dir[i][0];
            int nextc = node.bc + dir[i][1];
            int backr = node.br - dir[i][0];
            int backc = node.bc - dir[i][1];
            string ans = "";
            if( inMap(nextr,nextc) && inMap(backr,backc)
            && map[nextr][nextc]!='#' && map[backr][backc]!='#'
            && !visBox[nextr][nextc] ) {
                if(bfs2(node.pr, node.pc, backr, backc, node.br, node.bc, ans)) {
                    tmpnode.pr = node.br;
                    tmpnode.pc = node.bc;
                    tmpnode.br = nextr;
                    tmpnode.bc = nextc;
                    tmpnode.ans = node.ans +ans +pushes[i];
                    Q.push(tmpnode);
                }
            }
        }
    }
    return false;
}
int main() {
    int cas = 1;
    int sr , sc , br , bc;
    while(~scanf("%d%d",&R,&C) && R) {
        for(int i=0;i<R;i++) scanf("%s",map[i]);
        for(int i=0;i<R;i++)
            for(int j=0;j<C;j++) {
                if(map[i][j]=='S') {
                    sr = i; sc = j;
                }
                if(map[i][j]=='B') {
                    br = i; bc = j;
                }
            }
        path = "";
        printf("Maze #%d\n",cas++);
        bfs1(sr,sc,br,bc) ? cout<<path<<endl : cout<<"Impossible."<<endl;
        cout<<endl;
    }
    return 0;
}