题解：
有个小trick，代码里有注释
//by sdfzchy
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
typedef long long LL;
const int inf=(1<<30),N=401000;
int n,k;
inline int in()
{char ch=getchar();int f=1,tmp=0;while(ch<'0'||ch>'9') {
if(ch=='-') f=-1;ch=getchar();}while(ch>='0'&&ch<='9') {tmp=(tmp<<1)+(tmp<<3)+(ch-'0');ch=getchar();}return tmp*f;
}
struct E
{int to,nxt,dis;
}e[N<<1];
int head[N],ecnt;
void add(int u,int v,int w)
{++ecnt; e[ecnt]=(E){v,head[u],w}; head[u]=ecnt;++ecnt; e[ecnt]=(E){u,head[v],w}; head[v]=ecnt;
}int ans=inf;
int d[N],root,siz[N],dis[N],sum,dep[N];
bool vis[N];void dfs_root(int u,int F)
{siz[u]=1; d[u]=0;for(int i=head[u];i;i=e[i].nxt){int v=e[i].to;if(vis[v]||v==F) continue;dfs_root(v,u);siz[u]+=siz[v];d[u]=max(d[u],siz[v]);}d[u]=max(d[u],sum-siz[u]);if(d[u]<d[root]) root=u;
}int sta[N],top;
int ma[2001000];
int sta2[N],top2;void dfs_dep(int u,int F)
{sta[++top]=u;sta2[++top2]=u;if(k-dis[u]>=0)ans=min(ans,dep[u]+ma[k-dis[u]]);for(int i=head[u];i;i=e[i].nxt){int v=e[i].to;if(v==F||vis[v]) continue;dis[v]=dis[u]+e[i].dis;dep[v]=dep[u]+1;dfs_dep(v,u);}
}void dfs_sol(int u)
{vis[u]=1; ma[0]=0;// ma[0]=0!!!!!!!!!!! 不写就WA while(top2){if(dis[sta2[top2]]>k) top2--;else ma[dis[sta2[top2--]]]=inf;}top2=0;for(int i=head[u];i;i=e[i].nxt){int v=e[i].to;if(vis[v]) continue; dis[v]=e[i].dis; dep[v]=1; top=0;dfs_dep(v,0);ma[0]=0;while(top){int w=dis[sta[top]],v=dep[sta[top]];if(w>k) {top--;continue;}if(ma[w]>v) ma[w]=v;top--; }}for(int i=head[u];i;i=e[i].nxt){int v=e[i].to;if(!v||vis[v]) continue;root=0;sum=siz[v];ma[0]=0;dfs_root(v,u);dfs_sol(root);}
}int main()
{n=in(),k=in();for(int i=1,u,v,w;i<n;i++) u=in(),v=in(),w=in(),add(u+1,v+1,w); memset(ma,0x3f,sizeof(ma));d[0]=inf,root=0,sum=n;dfs_root(1,0);dfs_sol(root);if(ans>n+10) ans=-1;printf("%d\n",ans);return 0;
}