BZOJ 1189 【HNOI2007】 紧急疏散evacuate

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define File(s) freopen(s".in","r",stdin),freopen(s".out","w",stdout)
#define rep(i,l,r) for(int i=l;i<=r;i++)
#define INF 1000000000
#define maxn 200010
#define maxm 1000010
#define N 24

using namespace std;
typedef long long llg;

int zx[4]={1,-1,0,0},zy[4]={0,0,-1,1};
int n,m,nu[N][N],f[N*N][N*N],dor,tol,dep[maxn],d[maxn];
int head[maxn],next[maxm],to[maxm],c[maxm],tt,S,T;
char s[N][N]; bool w[N*N];

void link(int x,int y,int z){
	to[++tt]=y;next[tt]=head[x];head[x]=tt;
	to[++tt]=x;next[tt]=head[y];head[y]=tt;
	c[tt-1]=z; c[tt]=0;
}

bool bfs(){
    for(int i=1;i<=T;i++) dep[i]=-1;
    int ld=0,rd=0; dep[d[rd++]=S]=1;
    while(ld!=rd){
        int u=d[ld++];
        for(int i=head[u],v;v=to[i],i;i=next[i])
            if(c[i] && dep[v]==-1) dep[v]=dep[u]+1,d[rd++]=v;
    }
    return dep[T]!=-1;
}
  
int dfs(int u,int now){
    if(!now) return 0;
    if(u==T) return now;
    int low=0,res;
    for(int i=head[u],v;v=to[i],i;i=next[i])
        if(c[i] && dep[v]==dep[u]+1){
            res=dfs(v,min(now,c[i])); low+=res;
            c[i]-=res; c[i^1]+=res; now-=res;
        }
    if(!low) dep[u]=-1;
    return low;
}

bool check(int x){
	tt=1; S=tol*x+1; T=S+1;
	for(int i=1;i<=tol;i++)
		if(w[i])
			for(int j=1;j<=x;j++){
				link(tol*(j-1)+i,T,1);
				if(j!=x) link(tol*(j-1)+i,tol*j+i,INF);
			}
		else{
			link(S,i,1);
			for(int j=1;j<=tol;j++)
				if(w[j] && f[i][j]<=x) link(i,tol*(f[i][j]-1)+j,1);
		}
	int now=0; while(bfs()) now+=dfs(S,INF);
	for(int i=1;i<=T;i++) head[i]=0;
	return now==tol-dor;
}

int dx[N*N],dy[N*N]; bool vis[N][N];
void getdis(int x,int y){
	int u=nu[x][y]; if(w[u]) return;
	rep(i,1,n) rep(j,1,m) vis[i][j]=0;
	int ld=0,rd=0; dx[rd]=x,dy[rd++]=y; vis[x][y]=1;
	while(ld!=rd){
		int o=dx[ld],p=dy[ld++];
		for(int k=0,i,j,v;k<4;k++){
			i=o+zx[k],j=p+zy[k]; v=nu[i][j];
			if(i>=1 && i<=n && j>=1 && j<=m){
				if(vis[i][j]) continue;
				if(s[i][j]!=‘X‘) f[u][v]=f[u][nu[o][p]]+1;
				if(s[i][j]==‘.‘) dx[rd]=i,dy[rd++]=j,vis[i][j]=1;
			}
		}
	}
}

int main(){
	File("a");
	scanf("%d %d",&n,&m);
	for(int i=1;i<=n;i++){
		scanf("%s",s[i]+1);
		for(int j=1;j<=m;j++)
			if(s[i][j]!=‘X‘){
				nu[i][j]=++tol;
				dor+=(w[tol]=(s[i][j]==‘D‘));
			}
	}
	rep(i,1,tol) rep(j,1,tol) if(i!=j) f[i][j]=INF;
	rep(i,1,n) rep(j,1,m) if(nu[i][j]) getdis(i,j);
	int l=0,r=tol-dor+1,mid;
	while(l!=r){
		mid=(l+r)>>1;
		if(check(mid)) r=mid;
		else l=mid+1;
	}
	if(l>tol-dor) printf("impossible");
	else printf("%d",l);
	return 0;
}