真的调了好久！

#include<bits/stdc++.h>
using namespace std;
const int N=1e5+5;
int n,x,y,sz[N],dep[N],fa[N],w[N],son[N],top[N],tot,rev[N],id[N];
int mx[4*N];
string opt;
vector<pair<int,int> >edge[N];
struct Edge{
	int u,v,w;
}e[N];
void dfs1(int u){
	sz[u]=1;
	for(auto i:edge[u]){
		int v=i.first;
		if(v!=fa[u]){
			w[v]=i.second;
			dep[v]=dep[u]+1;
			fa[v]=u;
			dfs1(v);
			sz[u]+=sz[v];
			if(sz[v]>sz[son[u]]) son[u]=v;
		}
	}
}
void dfs2(int u,int t){
	top[u]=t;
	rev[++tot]=w[u];
	id[u]=tot;
	if(!son[u]) return;
	dfs2(son[u],t);
	for(auto i:edge[u]){
		int v=i.first;
		if(v!=fa[u]&&v!=son[u]) dfs2(v,v);
	}
}
void build(int x,int l,int r){
	if(l==r){
		mx[x]=rev[l];
		return;
	}
	int mid=(l+r)/2;
	build(2*x,l,mid);
	build(2*x+1,mid+1,r);
	mx[x]=max(mx[2*x],mx[2*x+1]);
}
void modify(int x,int l,int r,int h,int v){
	if(l==r){
		mx[x]=v;
		return;
	}
	int mid=(l+r)/2;
	if(h<=mid) modify(2*x,l,mid,h,v);
	else modify(2*x+1,mid+1,r,h,v);
	mx[x]=max(mx[2*x],mx[2*x+1]);
}
int query(int x,int l,int r,int L,int R){
	if(l>=L&&r<=R) return mx[x];
	int mid=(l+r)/2,ret=INT_MIN;
	if(L<=mid) ret=max(ret,query(2*x,l,mid,L,R));
	if(R>mid) ret=max(ret,query(2*x+1,mid+1,r,L,R));
	return ret;
}
void modify_point(int x,int v){
	modify(1,1,n,id[x],v);
}
int query_path(int x,int y){
	int ret=INT_MIN;
	while(dep[top[x]]!=dep[top[y]]){
		if(dep[top[x]]<dep[top[y]]) swap(x,y);
		ret=max(ret,query(1,1,n,id[top[x]],id[x]));
		x=fa[top[x]];
	}
	if(dep[x]>dep[y]) swap(x,y);
	ret=max(ret,query(1,1,n,id[x]+1,id[y]));
	return ret;
}
int main(){
	cin>>n;
	for(int i=1,u,v,w;i<n;i++){
		cin>>u>>v>>w;
		edge[u].push_back({v,w});
		edge[v].push_back({u,w});
		e[i]={u,v,w};
	}
	dfs1(1);
	dfs2(1,1);
	build(1,1,n);
	while(1){
		cin>>opt;
		if(opt=="CHANGE"){
			cin>>x>>y;
			x=(dep[e[x].u]>dep[e[x].v])?e[x].u:e[x].v;
			modify_point(x,y);
		}
		else if(opt=="QUERY"){
			cin>>x>>y;
			cout<<query_path(x,y)<<endl;
		}
		else break;
	}
	return 0;
}