51nod 1766 树上的最远点对（线段树）

#include<iostream> 
#include<cstring>
#include<cstdlib>
#include<cstdio>
#include<cmath> 
#include<algorithm> 
#define ll long long
using namespace std;
const int maxn=500010,inf=1e9;
struct tjm{int too,dis,pre;}e[maxn];
struct poi{int p[3];ll dis;}tree[maxn<<2];
int n,m,a,b,x,y,z,tot;
int d[maxn],son[maxn],size[maxn],fa[maxn],top[maxn],last[maxn];
ll dep[maxn];
inline void read(int &k)
{
    int f=1;k=0;char c=getchar();
    while(c<‘0‘||c>‘9‘)c==‘-‘&&(f=-1),c=getchar();
    while(c<=‘9‘&&c>=‘0‘)k=k*10+c-‘0‘,c=getchar();
    k*=f;
}
void add(int x,int y,int z){e[++tot].too=y;e[tot].dis=z;e[tot].pre=last[x];last[x]=tot;}
void dfs1(int x)
{
    size[x]=1;d[x]=d[fa[x]]+1;
    for(int i=last[x],too=e[i].too;i;i=e[i].pre,too=e[i].too)
    if(too!=fa[x])
    {   
        dep[too]=dep[x]+e[i].dis;
        fa[too]=x;dfs1(too);
        size[x]+=size[too];
        if(size[too]>size[son[x]])son[x]=too;
    }
}
void dfs2(int x,int tp)
{
    top[x]=tp;
    if(son[x])dfs2(son[x],tp);
    for(int i=last[x],too=e[i].too;i;i=e[i].pre,too=e[i].too)
    if(too!=fa[x]&&too!=son[x])dfs2(too,too);
}
int lca(int x,int y)
{
    int f1=top[x],f2=top[y];
    while(f1!=f2)
    {
        if(d[f1]<d[f2])swap(x,y),swap(f1,f2);
        x=fa[f1];f1=top[x];
    }
    if(d[x]<d[y])swap(x,y);
    return y;
}
void pushup(poi x,poi y,ll &dist,int &p1,int &p2)
{
    if(x.dis>y.dis)dist=x.dis,p1=x.p[1],p2=x.p[2];
    else dist=y.dis,p1=y.p[1],p2=y.p[2];
    for(int i=1;i<=2;i++)
    for(int j=1;j<=2;j++)
    if(x.p[i]&&y.p[j])
    {
        ll dis=dep[x.p[i]]+dep[y.p[j]]-(dep[lca(x.p[i],y.p[j])]<<1);
        if(dis>dist)dist=dis,p1=x.p[i],p2=y.p[j];
    }
}
void build(int x,int l,int r)
{
    if(l==r){tree[x].p[1]=tree[x].p[2]=l;return;}
    int mid=(l+r)>>1;
    build(x<<1,l,mid);build(x<<1|1,mid+1,r);
    pushup(tree[x<<1],tree[x<<1|1],tree[x].dis,tree[x].p[1],tree[x].p[2]);
}
void query(int x,int l,int r,int cl,int cr,ll &dis,int &p1,int &p2)
{
    if(cl<=l&&r<=cr){dis=tree[x].dis;p1=tree[x].p[1];p2=tree[x].p[2];return;}
    int mid=(l+r)>>1;
    poi t1,t2;t1.dis=t2.dis=-1;t1.p[1]=t1.p[2]=t2.p[1]=t2.p[2]=0;
    if(cl<=mid)query(x<<1,l,mid,cl,cr,t1.dis,t1.p[1],t1.p[2]);
    if(cr>mid)query(x<<1|1,mid+1,r,cl,cr,t2.dis,t2.p[1],t2.p[2]);
    pushup(t1,t2,dis,p1,p2);
}
int main()
{
    read(n);
    for(int i=1;i<n;i++)read(x),read(y),read(z),add(x,y,z),add(y,x,z);
    dfs1(1);dfs2(1,1);
    build(1,1,n);
    read(m);
    for(int i=1;i<=m;i++)
    {
        read(a),read(b),read(x),read(y);
        poi t1,t2;t1.dis=t2.dis=t1.p[1]=t1.p[2]=t2.p[1]=t2.p[2]=0;ll dis;int p1,p2;
        query(1,1,n,a,b,dis,t1.p[1],t1.p[2]);query(1,1,n,x,y,dis,t2.p[1],t2.p[2]);
        pushup(t1,t2,dis,p1,p2);
        printf("%lld\n",dis);
    }
}