# poj1475/acwing174 push box

Keywords: less

The idea of this question is quite new, but it is not so disgusting compared with the realization

Because it is required to move the box less, bfs can be used. Triple (x,y,dir) is used to indicate that the box is in (x,y). dir is the direction of the box where people are
Because people are required to move as little as possible on the basis of fewer box movements, each time people are judged to move, another bfs is needed (that is, a bfs is set in bfs)

In practice, there are six volumes stored in my queue
q[x][0]:dir
q[x][1]:x
q[x][2]:y
q[x][3]: last state (because output path is last required)
q[x][4]: number of steps the box is currently moving
q[x][5]: the number of steps a person currently moves

Pay attention to the disgusting point: the first bfs to (x,y,dir) is not necessarily the optimal state according to the number of boxes and the number of people. For the priority requirements of "NSWE" on acwing, it is also necessary to consider whether the pushed direction is suitable (and pay attention to judge the current corresponding direction according to "EWSN", because the direction of the previous step is relatively backward It's the top priority, the bottom priority is not so priority, visible code)

Code

```#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<queue>

using namespace std;
int n,m;
int sa,sb,ba,bb,ea,eb;
char s[25][25];
int dx[4]={1,-1,0,0},dy[4]={0,0,1,-1};
int lt[4]={'n'-'a','s'-'a','w'-'a','e'-'a'};
bool vis[25][25];
int vis2[25][25][4];
string str[25][25];
string bfs(int x,int y,int a,int b)
{
if(!(x>=1&&x<=n&&y>=1&&y<=m&&a>=1&&a<=n&&b>=1&&b<=m))return "!";
if(x==a&&y==b)return "";
memset(vis,0,sizeof(vis));
queue<pair<int,int> >qq;
while(!qq.empty())qq.pop();
qq.push(make_pair(x,y));
str[x][y]="";
while(!qq.empty())
{
int u=qq.front().first,v=qq.front().second;
qq.pop();
for(int i=0;i<4;++i)
{
int p=i;
i=i^1;
int tx=u+dx[i],ty=v+dy[i];
if(tx>=1&&tx<=n&&ty>=1&&ty<=m&&s[tx][ty]!='#'&&!vis[tx][ty])
{
str[tx][ty]=str[u][v]+char(lt[i^1]+'a');
if(tx==a&&ty==b)return str[tx][ty];
vis[tx][ty]=true;
qq.push(make_pair(tx,ty));
}
i=p;
}
}
return "!";
}
void trace(int x)
{
if(q[x][3]==-1)
{
s[ba][bb]='#';
cout<<bfs(sa,sb,q[x][1]+dx[q[x][0]],q[x][2]+dy[q[x][0]]);
s[ba][bb]='.';
return;
}
int k=q[x][3];
trace(k);
s[q[k][1]][q[k][2]]='#';
cout<<bfs(q[k][1]+dx[q[k][0]],q[k][2]+dy[q[k][0]],q[k][1]+dx[q[x][0]],q[k][2]+dy[q[x][0]]);
s[q[k][1]][q[k][2]]='.';
putchar(lt[q[x][0]]+'A');
}
void bfs2()
{
{
for(int i=0;i<4;++i)
{
int tx=x+dx[i],ty=y+dy[i];
int px=x-dx[i],py=y-dy[i];
if(px>=1&&px<=n&&py>=1&&py<=m&&s[px][py]!='#')
{
s[x][y]='#';
if(bfs(x+dx[dir],y+dy[dir],tx,ty)=="!")
{
s[x][y]='.';
continue;
}
int len=bfs(x+dx[dir],y+dy[dir],tx,ty).size();
s[x][y]='.';
if(vis2[px][py][i])
{
int k=vis2[px][py][i];
{
continue;
}
}
vis2[px][py][i]=++tail;
}
}
}
int ans=-1;
for(int i=0;i<4;++i)
{
if(vis2[ea][eb][i])
{
int k=vis2[ea][eb][i];
//			printf("%c\n",lt[q[k][0]]+'A');
if(ans==-1||q[k][4]<q[ans][4]||q[k][4]==q[ans][4]&&q[k][5]<q[ans][5]||q[k][4]==q[ans][4]&&q[k][5]==q[ans][5]&&q[k][0]>q[ans][0])ans=k;
}
}
//	printf("%c\n",lt[q[ans][0]]+'A');
if(ans==-1)printf("Impossible.\n");
else trace(ans),printf("\n");
printf("\n");
}
int main()
{
//	freopen("pushing.in","r",stdin);
//	freopen("pushing.out","w",stdout);
int tot=0;
while(scanf("%d%d",&n,&m)&&n)
{
printf("Maze #%d\n",++tot);
for(int i=1;i<=n;++i)
{
//			i=i^1;
scanf("%s",s[i]+1);
for(int j=1;j<=m;++j)
{
if(s[i][j]=='S')sa=i,sb=j,s[i][j]='.';
else if(s[i][j]=='B')ba=i,bb=j,s[i][j]='.';
else if(s[i][j]=='T')ea=i,eb=j,s[i][j]='.';
}
}

memset(vis2,0,sizeof(vis2));
s[ba][bb]='#';
for(int i=0;i<4;++i)
{
int ta=ba+dx[i],tb=bb+dy[i];
if(s[ta][tb]=='#')continue;
if(bfs(sa,sb,ta,tb)!="!")
{
vis2[ba][bb][i]=++tail;
q[tail][0]=i,q[tail][1]=ba,q[tail][2]=bb,q[tail][3]=-1,q[tail][4]=0,q[tail][5]=bfs(sa,sb,ta,tb).size();
}
}
s[ba][bb]='.';
bfs2();
}
return 0;
}
```

Attached is a set of data, which I use very well when I do it myself

```input
1 7
SB....T
1 7
SB..#.T
7 11
###########
#T##......#
#.#.#..####
#....B....#
#.######..#
#.....S...#
###########
8 4
....
.##.
.#..
.#..
.#.B
.##S
....
###T
5 11
#T##......#
#.#.#..####
#....B....#
#.######..#
#.....S...#
4 5
##T..
.SB..
.#...
...##
7 8
........
.######.
.....T#.
#.#####.
#...BS..
#..#####
########
10 5
...##
.#.##
.#.##
.#.##
.#.##
.#.##
.#...
SB...
##...
##T..
1 3
STB
1 3
SBT
1 3
TBS
3 1
T
B
S
3 3
S..
.B.
T.#
12 14
S.#...........
.B.#..........
....#.........
#....#........
.#....#.......
..#....#......
...#....#.....
....#....#....
.....#....#...
......#....#..
.......#....#.
........#...T#
11 19
....#####..........
....#...#..........
....#...#..........
..###..B##.........
..#......#.........
###.#.##.#...######
#...#.##.#####T...#
#.................#
####..###.#S##....#
...#......#########
...########........
20 20
S...................
.##################.
..................#.
.#.#..............#.
.#.###########.#..#.
.#.#...........#..#.
.#.#..########.#..#.
.#.#.........#.#..#.
.#.########..#.#..#.
.#.#.....B...#.#....
.#.#.######..#.#..#.
.#.#.........#.#..#.
.#.#..########.#..#.
.#.#............#.#.
.#.#............#.#.
.#.#.############.#.
..................#.
.#................#.
.##################.
...................T
20 20
....................
.##################.
..................#.
#################.#.
................#.#.
.##############.#.#.
.#............#.#.#.
.#.##########.#.#.#.
.#.#........#.#.#.#.
.#.#.####...#.#.#.#.
.#.#.#..SB..#.#.#.#.
.#.#.#.##T..#.#.#.#.
.#.#.#.######.#.#.#.
.#.#.#........#.#.#.
.#.#.##########.#.#.
.#.#............#.#.
.#.##############.#.
.#................#.
.##################.
....................
20 20
....................
.##################.
..................#.
#################.#.
................#.#.
.##############.#.#.
.#............#.#.#.
.#.##########.#.#.#.
.#.#........#.#.#.#.
.#.#.####...#.#.#.#.
.#.#.#.SB...#.#.#.#.
.#.#.#..#.###.#.#.#.
.#.#.#.....##.#.#.#.
.#.#.#.T.#....#.#.#.
.#.#.##########.#.#.
.#.#............#.#.
.#.##############.#.
.#................#.
.##################.
....................
20 20
#################..#
SB.................#
################.#.#
.................#.#
..############.#.#.#
#..............#.#.#
#.#.########.#.#.#.#
#.#..........#.#.#.#
#.#.#.####.#.#.#.#.#
#.#.#......#.#.#.#.#
#.#.#.#.##.#.#.#.#.#
#.#.#.#.##T#.#.#.#.#
#.#.#.#........#.#.#
#.#.#.#.######.#.#.#
#.#.#............#.#
#.#.#.##########.#.#
#.#................#
#.#.##############..
#...................
#..#################
20 20
..#.................
.#SB.#...........##.
.#.#.#.#.#####.###..
.#.....#.....#...#..
.#.###.#...#.....#.#
.#...#####.#####.#..
.....#.....#.#...##.
.###.#.#...#.#......
.#...#.#.###.#.####.
.#.....#.....#...#..
.#.###.#...#.....#.#
.#...#####.#####.#..
.....#.....#.#...##.
.###.#.#...#.#......
.#...#.#.###.#.###.#
.#.....#.....#...#..
.#.###.#...#.....##.
.#...#####.#####.#..
...........#.......#
.#########...####..T
20 20
S...................
.T..................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
..................B.
....................
20 20
T...................
.S..................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
...................B
20 20
####################
#SB#################
####################
####################
####################
####################
####################
####################
####################
####################
####################
####################
####################
####################
####################
####################
####################
####################
####################
###################T
20 1
S
.
B
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
T
20 3
S..
##.
...
.##
...
##.
...
.##
...
##.
...
.##
...
##.
...
.##
...
##.
...
.BT
1 20
TB.................S
0 0

```

Run my program if output

Published 27 original articles, won praise 1, and visited 397

Posted by Otiose Dodge on Wed, 12 Feb 2020 20:33:59 -0800