人作X部；把门按时间拆点，作Y部；如果某人能在某个时间到达某门则连边。就是个二分图最大匹配。
时间可以二分枚举，或者直接从1枚举时间然后加新边在原来的基础上进行增广。
谨记：时间是个不可忽视的维度。
#include<cstdio>
#include<cstring>
#include<queue>
#include<algorithm>
using namespace std;
#define INF (1<<30)
#define MAXN 80000
#define MAXM 80000*400 struct Edge{
int v,cap,flow,next;
}edge[MAXM];
int vs,vt,NE,NV;
int head[MAXN]; void addEdge(int u,int v,int cap){
edge[NE].v=v; edge[NE].cap=cap; edge[NE].flow=;
edge[NE].next=head[u]; head[u]=NE++;
edge[NE].v=u; edge[NE].cap=; edge[NE].flow=;
edge[NE].next=head[v]; head[v]=NE++;
} int level[MAXN];
int gap[MAXN];
void bfs(){
memset(level,-,sizeof(level));
memset(gap,,sizeof(gap));
level[vt]=;
gap[level[vt]]++;
queue<int> que;
que.push(vt);
while(!que.empty()){
int u=que.front(); que.pop();
for(int i=head[u]; i!=-; i=edge[i].next){
int v=edge[i].v;
if(level[v]!=-) continue;
level[v]=level[u]+;
gap[level[v]]++;
que.push(v);
}
}
} int pre[MAXN];
int cur[MAXN];
int ISAP(){
bfs();
memset(pre,-,sizeof(pre));
memcpy(cur,head,sizeof(head));
int u=pre[vs]=vs,flow=,aug=INF;
gap[]=NV;
while(level[vs]<NV){
bool flag=false;
for(int &i=cur[u]; i!=-; i=edge[i].next){
int v=edge[i].v;
if(edge[i].cap!=edge[i].flow && level[u]==level[v]+){
flag=true;
pre[v]=u;
u=v;
//aug=(aug==-1?edge[i].cap:min(aug,edge[i].cap));
aug=min(aug,edge[i].cap-edge[i].flow);
if(v==vt){
flow+=aug;
for(u=pre[v]; v!=vs; v=u,u=pre[u]){
edge[cur[u]].flow+=aug;
edge[cur[u]^].flow-=aug;
}
//aug=-1;
aug=INF;
}
break;
}
}
if(flag) continue;
int minlevel=NV;
for(int i=head[u]; i!=-; i=edge[i].next){
int v=edge[i].v;
if(edge[i].cap!=edge[i].flow && level[v]<minlevel){
minlevel=level[v];
cur[u]=i;
}
}
if(--gap[level[u]]==) break;
level[u]=minlevel+;
gap[level[u]]++;
u=pre[u];
}
return flow;
} int n,m;
char map[][];
int d[][];
bool vis[];
int dx[]={,,,-};
int dy[]={,-,,};
void bfs(int x,int y){
int s=x*m+y;
queue<int> que;
que.push(s);
while(!que.empty()){
x=que.front()/m; y=que.front()%m;
for(int i=; i<; ++i){
int nx=x+dx[i],ny=y+dy[i];
if( nx<||nx>=n||ny<||ny>=m || map[nx][ny]!='.' || d[s][nx*m+ny] ) continue;
d[s][nx*m+ny]=d[s][que.front()]+;
vis[nx*m+ny]=;
que.push(nx*m+ny);
}
que.pop();
}
}
bool canEscape(){
for(int i=; i<n; ++i){
for(int j=; j<m; ++j){
if(map[i][j]=='.' && !vis[i*m+j]) return ;
}
}
return ;
}
int escape(int time){
for(int i=; i<n; ++i){
for(int j=; j<m; ++j){
if(map[i][j]!='D') continue;
addEdge(i*m+j+time*n*m,vt,);
for(int x=; x<n; ++x){
for(int y=; y<m; ++y){
if(map[x][y]!='.') continue;
if(d[i*m+j][x*m+y] && d[i*m+j][x*m+y]<=time+) addEdge(x*m+y,i*m+j+time*n*m,);
}
}
}
}
return ISAP();
}
int main(){
int t;
scanf("%d",&t);
while(t--){
scanf("%d%d",&n,&m);
for(int i=; i<n; ++i) scanf("%s",map[i]);
memset(d,,sizeof(d));
memset(vis,,sizeof(vis));
for(int i=; i<n; ++i){
for(int j=; j<m; ++j){
if(map[i][j]=='D') bfs(i,j);
}
}
if(!canEscape()){
puts("impossible");
continue;
} vs=; vt=vs+; NV=vt+; NE=;
memset(head,-,sizeof(head));
int tot=;
for(int i=; i<n; ++i){
for(int j=; j<m; ++j){
if(map[i][j]=='.') addEdge(vs,i*m+j,),++tot;
}
}
int time=;
while(tot){
tot-=escape(time);
++time;
}
printf("%d\n",time);
}
}