题目：
Vivek has encountered a problem. He has a maze that can be represented as an n×m grid. Each of the grid cells may represent the following:
Empty — ‘.’
Wall — ‘#’
Good person — ‘G’
Bad person — ‘B’
The only escape from the maze is at cell (n,m).
A person can move to a cell only if it shares a side with their current cell and does not contain a wall. Vivek wants to block some of the empty cells by replacing them with walls in such a way, that all the good people are able to escape, while none of the bad people are able to. A cell that initially contains ‘G’ or ‘B’ cannot be blocked and can be travelled through.
Help him determine if there exists a way to replace some (zero or more) empty cells with walls to satisfy the above conditions.
It is guaranteed that the cell (n,m) is empty. Vivek can also block this cell.
Input
The first line contains one integer t (1≤t≤100) — the number of test cases. The description of the test cases follows.
The first line of each test case contains two integers n, m (1≤n,m≤50) — the number of rows and columns in the maze.
Each of the next n lines contain m characters. They describe the layout of the maze. If a character on a line equals ‘.’, the corresponding cell is empty. If it equals ‘#’, the cell has a wall. ‘G’ corresponds to a good person and ‘B’ corresponds to a bad person.
Output
For each test case, print “Yes” if there exists a way to replace some empty cells with walls to satisfy the given conditions. Otherwise print “No”
You may print every letter in any case (upper or lower).
Example
Input
6
1 1
.
1 2
G.
2 2
#B
G.
2 3
G.#
B#.
3 3
#B.
#…
GG.
2 2
#B
B.
Output
Yes
Yes
No
No
Yes
Yes
Note
For the first and second test cases, all conditions are already satisfied.
For the third test case, there is only one empty cell (2,2), and if it is replaced with a wall then the good person at (1,2) will not be able to escape.
For the fourth test case, the good person at (1,1) cannot escape.
For the fifth test case, Vivek can block the cells (2,3) and (2,2).
For the last test case, Vivek can block the destination cell (2,2).
题解：
bfs
如果遇到坏人就把他四周的路都堵上
如果坏人周围有好人，则一定不行
可以从n,m出发，寻找能够到达的点，如果到达G的个数等于原本G的个数就可以，否则不可以
#include <bits/stdc++.h>
using namespace std;
char pos[55][55];
int dx[4]={0,0,1,-1};
int dy[4]={1,-1,0,0};
int vis[55][55];
int r=1;
int n,m;
struct node {
int x,y;
};
void check(int x,int y)
{
for(int i=0;i<4;i++)
{
int nx=x+dx[i];
int ny=y+dy[i];
if(nx>=1&&nx<=n&&ny>=1&&ny<=m)
{
if(pos[nx][ny]=='.') pos[nx][ny]='#';
else if(pos[nx][ny]=='G') r=0;
}
}
}
void bfs(int a,int b)
{
queue<node> q;
vis[a][b]=1;
struct node now,next;
now.x=a,now.y=b;
q.push(now);
while(!q.empty())
{
now=q.front();
q.pop();
for(int i=0;i<4;i++)
{
next.x=now.x+dx[i];
next.y=now.y+dy[i];
if(next.x<=0||next.x>n||next.y<=0||next.y>m||pos[next.x][next.y]=='#'||vis[next.x][next.y])
{
continue;
}
vis[next.x][next.y]=1;
q.push(next);
}
}
}
int main()
{
int t;
cin>>t;
while(t--)
{
r=1;
scanf("%d %d",&n,&m);
for(int i=1;i<=n;i++)
{
for(int j=1;j<=m;j++)
{
cin>>pos[i][j];
}
}
for(int i=1;i<=n;i++)
{
for(int j=1;j<=m;j++)
{
if(pos[i][j]=='B')
{
check(i,j);
}
}
}
memset(vis,0,sizeof(vis));
if(pos[n][m]!='#') bfs(n,m);
for(int i=1;i<=n;i++)
{
for(int j=1;j<=m;j++)
{
if(pos[i][j]=='G')
{
if(vis[i][j]==0)
{
r=0;
break;
}
}
}
}
if(r) cout<<"YES"<<endl;
else cout<<"NO"<<endl;
}
return 0;
}