1 #include <cmath> 2 #include <cstdio> 3 #include <cstring> 4 #include <cstring> 5 #include <iostream> 6 #include <algorithm> 7 # define maxn 300010 8 using namespace std; 9 void ot(){cout<<"****"<<endl;} 10 int n; 11 int m; 12 struct node{ 13 int u,v,w,nxt; 14 }g[2*maxn]; 15 int adj[maxn],e; 16 void add(int u,int v,int w){ 17 g[e]=(node){u,v,w,adj[u]}; 18 adj[u]=e++; 19 } 20 int pre[maxn],dis[maxn]; 21 bool vis[maxn],pd[maxn]; 22 int q[maxn],head,tail; 23 int st,ed,mx_dis,mn_dis; 24 int lin[maxn],top; 25 void beg(){ 26 head=tail=1; 27 q[tail++]=1; vis[1]=1; 28 int mx=0; 29 while(head<tail){ 30 int k=q[head++]; 31 for(int i=adj[k];i!=-1;i=g[i].nxt){ 32 int v=g[i].v; if(vis[v]) continue; 33 dis[v]=dis[k]+g[i].w; vis[v]=1; 34 if(dis[v]>mx) mx=dis[v],st=v; 35 q[tail++]=v; 36 } 37 } 38 memset(vis,0,sizeof(vis)); 39 memset(dis,0,sizeof(dis)); 40 head=tail=1; mx=0; 41 q[tail++]=st; vis[st]=1; 42 while(head<tail){ 43 int k=q[head++]; 44 for(int i=adj[k];i!=-1;i=g[i].nxt){ 45 46 int v=g[i].v; if(vis[v]) continue; 47 dis[v]=dis[k]+g[i].w; vis[v]=1; 48 pre[v]=k; 49 if(dis[v]>mx) mx=dis[v],ed=v; 50 q[tail++]=v; 51 } 52 } 53 mx_dis=mx; 54 int x=ed; 55 while(x){ 56 pd[x]=1; lin[++top]=dis[x]; 57 x=pre[x]; 58 } 59 memset(vis,0,sizeof(vis)); 60 memset(dis,0,sizeof(dis)); 61 head=tail=1; mx=0; 62 q[tail++]=ed; vis[ed]=1; 63 while(head<tail){ 64 int k=q[head++]; 65 for(int i=adj[k];i!=-1;i=g[i].nxt){ 66 int v=g[i].v; if(vis[v]) continue; 67 vis[v]=1; 68 if(pd[v]) dis[v]=dis[k]; 69 else dis[v]=dis[k]+g[i].w; 70 mx=max(mx,dis[v]); 71 q[tail++]=v; 72 } 73 } 74 mn_dis=mx; 75 } 76 bool check(int x){ 77 int l=1,r=top; 78 while(lin[1]-lin[l+1]<=x && l<=top) l++; 79 while(lin[r-1]-lin[top]<=x && r>=1) r--; 80 // cout<<"lr== "<<l<<" "<<r<<endl; 81 // cout<<lin[l]<<" * "<<lin[r]<<endl; 82 return lin[l]-lin[r]<=m?1:0; 83 } 84 int main(){ 85 // freopen("a.in","r",stdin); 86 scanf("%d%d",&n,&m); 87 int x,y,z; 88 memset(adj,-1,sizeof(adj)); 89 for(int i=1;i<n;i++){ 90 scanf("%d%d%d",&x,&y,&z); 91 add(x,y,z); add(y,x,z); 92 } 93 beg(); 94 int l=mn_dis,r=mx_dis,mid,ans; 95 // for(int i=1;i<=n;i++) cout<<pd[i]<<" "; cout<<endl; 96 // cout<<"top= "<<top<<endl; 97 // for(int i=1;i<=top;i++) cout<<lin[i]<<" "; cout<<endl; 98 // cout<<check(5)<<endl; exit(0); 99 while(l<=r){ 100 mid=(l+r)>>1; 101 if(check(mid)) ans=mid,r=mid-1; 102 else l=mid+1; 103 } 104 cout<<ans<<endl; 105 }