BZOJ 1189 HNOI2007 紧急疏散evacuate

#include <bits/stdc++.h>
#define ll long long
#define inf 1e9+10
#define p(x,y) (x-1)*m+y
using namespace std;
inline int read(){
	int x=0;int f=1;char ch=getchar();
	while(!isdigit(ch)) {if(ch==‘-‘) f=-1;ch=getchar();}
	while(isdigit(ch)) {x=x*10+ch-‘0‘;ch=getchar();}
	return x*f;
}
const int MAXN=1e5+10;
const int dx[4]={1,0,-1,0};
const int dy[4]={0,1,0,-1};
struct node{
	int y,next,flow,back;
}e[MAXN];
int linkk[MAXN],level[10000],q[MAXN],head,tail,len,s,t,d[MAXN][2],dis[500][25][25],n,m,sum,mp[30][30],door=1;
inline void insert(int x,int y,int f){
	//cout<<x<<‘ ‘<<y<<endl;
	e[++len].y=y;e[len].next=linkk[x];linkk[x]=len;e[len].flow=f;e[len].back=len+1;
	e[++len].y=x;e[len].next=linkk[y];linkk[y]=len;e[len].flow=0;e[len].back=len-1;
}
inline bool getlevel(){
	head=tail=0;
	memset(level,-1,sizeof(level));
	q[++tail]=s;level[s]=0;
	while(head<tail){
		int tn=q[++head];
		for(int i=linkk[tn];i;i=e[i].next){
			if(level[e[i].y]==-1&&e[i].flow){
				level[e[i].y]=level[tn]+1;
				q[++tail]=e[i].y;
			}
		}
	}
	return level[t]>=0;
}
inline int getmaxflow(int x,int flow){
	int f=0,d;
	if(x==t) return flow;
	for(int i=linkk[x];i;i=e[i].next){
		if(level[e[i].y]==level[x]+1&&e[i].flow){
			if(d=getmaxflow(e[i].y,min(e[i].flow,flow-f))){
				f+=d;e[i].flow-=d;e[e[i].back].flow+=d;
				if(f==flow) return f;
			}
		}
	}
	if(f==0) level[x]=-1;
	return f;
}
inline int dinic(){
	int ans=0,d;
	while(getlevel()){
		while(d=getmaxflow(s,inf)) ans+=d;
	}
	return ans;
}
inline void bfs(int k,int x,int y){
	head=tail=0;
	d[++tail][0]=x;d[tail][1]=y;
	dis[k][x][y]=0;
	while(head<tail){
		int xx=d[++head][0];int yy=d[head][1];
		for(int i=0;i<4;i++){
			int tx=xx+dx[i];int ty=yy+dy[i];
			if(tx<1||tx>n||ty<1||ty>m||mp[tx][ty]!=1||dis[k][tx][ty]!=inf) continue;
			dis[k][tx][ty]=dis[k][xx][yy]+1;
			d[++tail][0]=tx;d[tail][1]=ty;
		}
	}
}
inline bool check(int mid){
    s=0;t=n*m+1+mid*door;
	memset(linkk,0,sizeof(linkk));len=0;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
			if(mp[i][j]==1) insert(s,p(i,j),1);
		}
	}
	for(int i=2;i<=door;i++){
		for(int j=1;j<=mid;j++){
			insert(n*m+(i-2)*mid+j,t,1);
			if(j!=mid) insert(n*m+(i-2)*mid+j,n*m+(i-2)*mid+j+1,inf);
		}
	}
	for(int k=2;k<=door;k++){
		for(int i=1;i<=n;i++){
			for(int j=1;j<=m;j++){
				if(dis[k][i][j]<=mid&&mp[i][j]==1) insert(p(i,j),n*m+(k-2)*mid+dis[k][i][j],1);
			}
		}
	}
	int ans=dinic();
	//cout<<mid<<‘ ‘<<ans<<‘ ‘<<sum<<endl;
	return ans==sum;
}
int main(){
	n=read();m=read();
	for(int i=1;i<=n;i++){
		char ch[30];
		scanf("%s",ch+1);
		for(int j=1;j<=m;j++){
			if(ch[j]==‘X‘) mp[i][j]=0;
			if(ch[j]==‘.‘) mp[i][j]=1,sum++;
			if(ch[j]==‘D‘) mp[i][j]=++door;
		}
	}
	for(int i=2;i<=door;i++){
		for(int j=1;j<=n;j++){
			for(int k=1;k<=m;k++){
				dis[i][j][k]=inf;
			}
		}
	}
	for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
			if(mp[i][j]>1) bfs(mp[i][j],i,j);
		}
    }
    int l=0;int r=400;int mn=-1;
    while(l<=r){
    	int mid=(l+r)>>1;
    	if(check(mid)) r=mid-1,mn=mid;
    	else l=mid+1;
	}
	if(mn==-1) cout<<"impossible"<<endl;
	else cout<<mn<<endl;
	return 0;
}