RT,LCA写T了两个点，尽力卡了，然鹅并无甚用，请求支援。

#include<bits/stdc++.h>
using namespace std;
int n,m,s,dp[300005][20],d[300005],lg[300005],dis[300005],uuu[300005],vvv[300005],diff[300005],maxnn,maxn;
struct node{
	int v,w;
};
vector<node>nbr[300005];
int max(int a,int b){
	return (a>b)?a:b;
}
inline int read()
{
	int x=0,f=1;char ch=getchar();
	while (ch<'0'||ch>'9'){if (ch=='-') f=-1;ch=getchar();}
	while (ch>='0'&&ch<='9'){x=(x<<1)+(x<<3)+ch-48;ch=getchar();}
	return x*f;
}
void before(int cur,int fa){
	d[cur]=d[fa]+1;
	dp[cur][0]=fa;
	for(register int i=1;i<=lg[n];i++){
		dp[cur][i]=dp[dp[cur][i-1]][i-1];
	}
	for(register int i=0;i<nbr[cur].size();i++){
		int son=nbr[cur][i].v;
		if(son==fa)continue;
		dis[son]=dis[cur]+nbr[cur][i].w;
		before(son,cur);
	}
	return;
}
inline int LCA(int x,int y/*under*/){
	if(d[x]>d[y])swap(x,y);
	for(register int i=lg[n];i>=0;i--){
		if(d[x]<=d[y]-(1<<i))y=dp[y][i];
	}
	if(x==y)return x;
	for(register int i=lg[n];i>=0;i--){
		if(dp[x][i]!=dp[y][i]){
			x=dp[x][i];
			y=dp[y][i];
		}
	}
	return dp[x][0];
}
inline void dfs(int cur,int fa){
	for(register int i=0;i<nbr[cur].size();i++){
		int son=nbr[cur][i].v;
		if(son==fa)continue;
		dfs(son,cur);
		diff[cur]+=diff[son];
	}
}
inline void Get(int cur,int fa,int cnt){
	for(register int i=0;i<nbr[cur].size();i++){
		int son=nbr[cur][i].v;
		if(son==fa)continue;
		if(diff[son]==cnt){
			maxnn=max(maxnn,nbr[cur][i].w);
		}
		Get(son,cur,cnt);
	}
}
inline void clean(){
	for(register int i=1;i<=n;i++)diff[i]=0; 
}
inline bool check(int x){
	clean();
	int cnt=0;
	for(register int i=1;i<=m;i++){
		int l=LCA(uuu[i],vvv[i]);
		int len=dis[uuu[i]]+dis[vvv[i]]-(dis[l]<<1);
		if(len>x){
			cnt++;
			diff[uuu[i]]++;
			diff[vvv[i]]++;
			diff[l]-=2;
		}
	}
	if(maxn<=x)return 1;
	dfs(1,0);
	maxnn=0;
	Get(1,0,cnt);
	if(maxn-maxnn<=x)return 1;
	return 0;
}
int main(){
	n=read();m=read();
	lg[0]=-1;
	for(register int i=1;i<n;i++){
		int u=read(),v=read(),w=read();
		node bag1={v,w},bag2={u,w};
		nbr[u].push_back(bag1);
		nbr[v].push_back(bag2);
		lg[i]=lg[i>>1]+1;
		
	}
	lg[n]=lg[n>>1]+1;
	before(1,0);
	for(register int i=1;i<=m;i++){
		uuu[i]=read();
		vvv[i]=read();
	}
	for(int i=1;i<=m;i++){
		int l=LCA(uuu[i],vvv[i]);
		int len=dis[uuu[i]]+dis[vvv[i]]-(dis[l]<<1);
		maxn=max(maxn,len);
	}
	int lt=-1,rt=maxn+1;
	while(lt+1<rt){
		int mid=lt+rt>>1;
		if(check(mid))rt=mid;
		else lt=mid;
	}
	cout<<rt;
	return 0;
}