POJ 1130

#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
const int N=10000;
int head
,dist
,nc,_nc,_head
;
struct Edge
{
int to,next;
} edge[N*100],_edge[N*100];
void add(int a,int b)
{
edge[nc].to=b;
edge[nc].next=head[a];
head[a]=nc++;
}
void addf(int a,int b)
{
_edge[_nc].to=b;
_edge[_nc].next=_head[a];
_head[a]=_nc++;
}
bool vis
;
int T;
void diji(int n)
{
memset(dist,-1,sizeof(dist));
dist[T]=0;
memset(vis,false,sizeof(vis));
for(int i=0;i<n;i++)
{
int k=-1,mmin=1<<30;
for(int j=0;j<n;j++)
if(!vis[j]&&dist[j]!=-1&&mmin>dist[j])
mmin=dist[k=j];
if(k==-1)
return;
vis[k]=true;
for(int j=_head[k];j!=-1;j=_edge[j].next)
{
int t=_edge[j].to;
if(dist[t]==-1||dist[t]>dist[k]+1)
dist[t]=dist[k]+1;
}
}
}
bool check(int now)
{
vis[now]=true;
if(now==T)
return true;
for(int i=head[now];i!=-1;i=edge[i].next)
{
if(!vis[edge[i].to])
{
if(check(edge[i].to))
return true;
}
}
return false;
}
int main()
{
int n,a,b;
scanf("%d%d",&n,&T);
memset(head,-1,sizeof(head));
memset(_head,-1,sizeof(_head));
nc=_nc=0;
while(scanf("%d%d",&a,&b)!=EOF)
add(a,b),addf(b,a);
diji(n);
int ans=0;
for(int i=1; i<n; i++)
{
if(i!=T&&dist[i]!=-1&&dist[i]<dist[ans])
{
memset(vis,false,sizeof(vis));
vis[i]=true;
if(!check(0))
ans=i;
}
}
printf("Put guards in room %d.\n",ans);
return 0;
}