bzoj 1912 [Apio2010]patrol 巡逻 树形dp

#include <bits/stdc++.h>
using namespace std;
#define N 110000
int n,K,tot;
int f
,head
,nex[N<<1],to[N<<1];
int bj
,deep
,fa
,pos
;
int p1,p2,ans,fin;
void add(int x,int y)
{
tot++;
nex[tot]=head[x];head[x]=tot;
to[tot]=y;
}
void dfs(int x,int y)
{
deep[x]=deep[y]+1;
f[x]=0;pos[x]=x;fa[x]=y;
for(int i=head[x];i;i=nex[i])
if(to[i]!=y)
{
dfs(to[i],x);
if(f[x]+f[to[i]]+bj[to[i]]>ans)
{
ans=f[x]+f[to[i]]+bj[to[i]];
p1=pos[x];p2=pos[to[i]];
}
if(f[to[i]]+bj[to[i]]>f[x])
{
f[x]=f[to[i]]+bj[to[i]];
pos[x]=pos[to[i]];
}
}
}
int main()
{
//freopen("tt.in","r",stdin);
scanf("%d%d",&n,&K);
for(int i=1,x,y;i<n;i++)
{
scanf("%d%d",&x,&y);
add(x,y);add(y,x);
}
for(int i=1;i<=n;i++)bj[i]=1;
dfs(1,0);fin+=ans;
if(K==2)
{
if(deep[p1]<deep[p2])swap(p1,p2);
while(deep[p1]>deep[p2])
bj[p1]=-1,p1=fa[p1];
while(p1!=p2)
{
bj[p1]=-1;bj[p2]=-1;
p1=fa[p1];p2=fa[p2];
}
ans=-n;
dfs(1,0);fin+=ans;
}
printf("%d\n",2*(n-1)-fin+K);
return 0;
}