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| #include<algorithm>
#include<iostream>
#include<iomanip>
#include<cstring>
#include<cstdlib>
#include<climits>
#include<vector>
#include<cstdio>
#include<cmath>
#include<queue>
using namespace std;
inline const int Get_Int() {
int num=0,bj=1;
char x=getchar();
while(x<'0'||x>'9') {
if(x=='-')bj=-1;
x=getchar();
}
while(x>='0'&&x<='9') {
num=num*10+x-'0';
x=getchar();
}
return num*bj;
}
const int maxn=100005;
const double eps=1e-4;
double max(double a,double b) {
return a>b?a:b;
}
int id[maxn];
struct Segment_Tree {
struct Tree {
int left,right;
double max;
Tree(int l=0,int r=0,double m=-1e18):left(l),right(r),max(m) {}
} tree[maxn<<2];
#define ls index<<1
#define rs index<<1|1
void build(int index,int Left,int Right) {
tree[index]=Tree(Left,Right);
if(Left==Right) {
id[Left]=index;
return;
}
int mid=(Left+Right)>>1;
build(ls,Left,mid);
build(rs,mid+1,Right);
}
void refresh(int Now,double v) {
Now=id[Now];
while(Now) {
tree[Now].max=max(tree[Now].max,v);
Now>>=1;
}
}
double query(int index,int Left,int Right) {
if(Right<tree[index].left||Left>tree[index].right)return -1e18;
if(Left<=tree[index].left&&Right>=tree[index].right)return tree[index].max;
return max(query(ls,Left,Right),query(rs,Left,Right));
}
} st;
struct Edge {
int from,to;
double dist;
};
int n,L,R,step=0,Depth[maxn],Maxdep[maxn],Son[maxn],father[maxn],Top[maxn],Dfn[maxn];
double Sondist[maxn],tmp[maxn];
vector<Edge> edges[maxn];
void AddEdge(int x,int y,double v) {
edges[x].push_back((Edge) {x,y,v});
}
void Dfs1(int Now,int fa,int depth) {
Depth[Now]=Maxdep[Now]=depth;
father[Now]=fa;
for(Edge& e:edges[Now]) {
int Next=e.to;
if(Next==fa)continue;
Dfs1(Next,Now,depth+1);
if(Maxdep[Next]>Maxdep[Son[Now]]) {
Son[Now]=Next;
Sondist[Now]=e.dist;
Maxdep[Now]=Maxdep[Next];
}
}
}
void Dfs2(int Now,int top) {
Top[Now]=top;
Dfn[Now]=++step;
if(Son[Now])Dfs2(Son[Now],top);
for(Edge& e:edges[Now]) {
int Next=e.to;
if(Next==father[Now]||Next==Son[Now])continue;
Dfs2(Next,Next);
}
}
double Ask(int Now,int Left,int Right) {
Left=max(Left,0);
Right=min(Right,Maxdep[Now]-Depth[Now]);
if(Left>Right)return -1e18;
return st.query(1,Dfn[Now]+Left,Dfn[Now]+Right);
}
double ans=1e-8;
void TreeDp(int Now,double dist,double mid) {
st.refresh(Dfn[Now],dist);
if(Son[Now])TreeDp(Son[Now],dist+Sondist[Now]-mid,mid);
for(Edge& e:edges[Now]) {
int Next=e.to;
if(Next==father[Now]||Next==Son[Now])continue;
TreeDp(Next,dist+e.dist-mid,mid);
for(int j=1; j<=Maxdep[Next]-Depth[Next]+1; j++) {
tmp[j]=st.tree[id[Dfn[Next]+j-1]].max;
ans=max(ans,tmp[j]+Ask(Now,L-j,R-j)-2*dist);
}
for(int j=1; j<=Maxdep[Next]-Depth[Next]+1; j++)st.refresh(Dfn[Now]+j,tmp[j]);
}
ans=max(ans,Ask(Now,L,R)-dist);
}
bool Check(double mid) {
st.build(1,1,n);
ans=-1e18;
TreeDp(1,0,mid);
return ans>=-eps;
}
int main() {
int size=40<<20;
__asm__ ("movq %0,%%rsp\n"::"r"((char*)malloc(size)+size));
n=Get_Int();
L=Get_Int();
R=Get_Int();
for(int i=1; i<n; i++) {
int x=Get_Int(),y=Get_Int(),v=Get_Int();
AddEdge(x,y,v);
AddEdge(y,x,v);
}
Dfs1(1,0,1);
Dfs2(1,1);
double Left=0,Right=1e10;
while(Right-Left>eps) {
double mid=(Left+Right)/2;
if(Check(mid))Left=mid;
else Right=mid;
}
printf("%0.3lf\n",(Left+Right)/2);
exit(0);
}
|
