bzoj2599 [IOI2011]Race

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cmath>
#include<queue>
#include<deque>
#include<set>
#include<map>
#include<ctime>
#define LL long long
#define inf 0x7ffffff
#define pa pair<int,int>
#define pi 3.1415926535897932384626433832795028841971
#define N 200010
using namespace std;
inline LL read()
{
    LL x=0,f=1;char ch=getchar();
    while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}
    while(ch>=‘0‘&&ch<=‘9‘){x=x*10+ch-‘0‘;ch=getchar();}
    return x*f;
}
int n,k,cnt,sum,root,ans;
struct edge{
	int to,next,v;
}e[2*N];
int head[N];
int f[N];
int son[N];
int vis[N];
int dep[N];
int dist[N];
int t[1000010];
inline void ins(int u,int v,int w)
{
	e[++cnt].to=v;
	e[cnt].v=w;
	e[cnt].next=head[u];
	head[u]=cnt;
}
inline void insert(int u,int v,int w)
{
	ins(u,v,w);
	ins(v,u,w);
}
inline void getroot(int x,int fa)
{
	son[x]=1;f[x]=0;
	for (int i=head[x];i;i=e[i].next)
	  if (e[i].to!=fa&&!vis[e[i].to])
	  {
	  	getroot(e[i].to,x);
	  	son[x]+=son[e[i].to];
	  	f[x]=max(f[x],son[e[i].to]);
	  }
	f[x]=max(f[x],sum-son[x]);
	if (f[x]<f[root])root=x;
}
inline void dfs(int x,int fa)
{
	if(dist[x]<=k)ans=min(ans,dep[x]+t[k-dist[x]]);
	for (int i=head[x];i;i=e[i].next)
	  if (fa!=e[i].to&&!vis[e[i].to])
	  {
	  	dep[e[i].to]=dep[x]+1;
	  	dist[e[i].to]=dist[x]+e[i].v;
	  	dfs(e[i].to,x);
	  }
}
inline void add(int x,int fa,int opr)
{
	if (dist[x]<=k)
	{
		if (opr==1)t[dist[x]]=min(t[dist[x]],dep[x]);
		else t[dist[x]]=inf;
	}
	for (int i=head[x];i;i=e[i].next)
	  if (e[i].to!=fa&&!vis[e[i].to])
	    add(e[i].to,x,opr);
}
inline void work(int x)
{
	vis[x]=1;t[0]=0;
	for (int i=head[x];i;i=e[i].next)
	  if (!vis[e[i].to])
	  {
	  	dep[e[i].to]=1;dist[e[i].to]=e[i].v;
	  	dfs(e[i].to,0);
	  	add(e[i].to,0,1);
	  }
	for (int i=head[x];i;i=e[i].next)
	  if (!vis[e[i].to])add(e[i].to,0,0);
	for (int i=head[x];i;i=e[i].next)
	  if (!vis[e[i].to])
	  {
		root=0;sum=son[e[i].to];
		getroot(e[i].to,0);
		work(root);
	  }
}
int main()
{
	n=read();k=read();
	for (int i=1;i<=k;i++)t[i]=n;
	for(int i=1;i<n;i++)
	{
		int x=read()+1,y=read()+1,z=read();
		insert(x,y,z);
	}
	ans=sum=f[0]=n;
	getroot(1,0);
	work(root);
	if (ans==n)printf("-1\n");
	else printf("%d\n",ans);
	return 0;
}