样例未过，求条

#include<iostream>
#include<vector>
#include<algorithm>
#include<cstring>
#define debug cout<<__LINE__<<endl;
using namespace std;
int n,m,s,fa[510086][20],dep[510086],a[510086][2],k[510086],length[510086][20],mx[510086][20],ll[510086];
bool mark[510086];
vector<pair<int,int> > g[510086];
void dfs(int x,int f,int w){
    dep[x]=dep[f]+1;
    fa[x][0]=f;
    length[x][0]=mx[x][0]=w;
    for(int i=1;i<=19;i++){
        fa[x][i]=fa[fa[x][i-1]][i-1];
        length[x][i]=length[fa[x][i-1]][i-1]+length[x][i-1];
        mx[x][i]=max(mx[fa[x][i-1]][i-1],mx[x][i-1]);
    }
    for(pair<int,int> k:g[x]){
    	int v=k.first,w=k.second;
        if(v==f) continue;
        dfs(v,x,w);
    }
}
int solve(int x,int y){
    if(dep[x]<dep[y]) swap(x,y);
    while(dep[x]!=dep[y]){
        x=fa[x][int(log2(dep[x]-dep[y]))];
    }
    if(x==y) return x;
    for(int i=18;i>=0;i--){
        if(fa[x][i]!=fa[y][i]) x=fa[x][i],y=fa[y][i];
    }
    return fa[x][0];
}
void dfs2(int x,int f){
    for(pair<int,int> q:g[x]){
    	int v=q.first;
    	if(v==f) continue;
        dfs2(v,x);
        k[x]+=k[v];
    }
}
int len(int x,int y){
	int ans=0;
    if(dep[x]<dep[y]) swap(x,y);
    while(dep[x]!=dep[y]){
    	ans+=length[x][int(log2(dep[x]-dep[y]))];
        x=fa[x][int(log2(dep[x]-dep[y]))];
    }
    if(x==y) return ans;
    for(int i=18;i>=0;i--){
        if(fa[x][i]!=fa[y][i]) ans+=length[x][i]+length[y][i],x=fa[x][i],y=fa[y][i];
    }
    return ans+fa[x][0]+fa[y][0];
}
int lmx(int x,int y){
	int ans=0;
    if(dep[x]<dep[y]) swap(x,y);
    while(dep[x]!=dep[y]){
    	ans=max(ans,mx[x][int(log2(dep[x]-dep[y]))]);
        x=fa[x][int(log2(dep[x]-dep[y]))];
    }
    if(x==y) return ans;
    for(int i=18;i>=0;i--){
        if(fa[x][i]!=fa[y][i]) ans=max(ans,mx[x][i]+mx[y][i]),x=fa[x][i],y=fa[y][i];
    }
    return max({ans,fa[x][0],fa[y][0]});
}
bool check(int c){
	int t=0,mx=0;
	memset(mark,0,sizeof mark);
	memset(k,0,sizeof k);
	for(int i=1;i<=m;i++){
		if(len(a[i][0],a[i][1])>c){
			mark[i]=1;
			k[a[i][0]]++,k[a[i][1]]++;
			int lca=solve(a[i][0],a[i][1]);
			k[lca]-=2;
			t++;
			mx=max(mx,lmx(a[i][0],a[i][1]));
		}
	}
	if(t==0) return true;
	dfs2(1,0);
	int l=0;
	for(int i=1;i<n;i++){
		if(k[i]==t) l=max(l,ll[i]);
	}
	return mx-l<=c;
}
int main(){
    ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    cin>>n>>m;
    int x,y,t;
    for(int i=1;i<n;i++){
        cin>>x>>y>>t;
        g[x].push_back({y,t});
        g[y].push_back({x,t});
        ll[i]=t;
    }
    dfs(1,0,0);
    for(int i=1;i<=m;i++){
        cin>>a[i][0]>>a[i][1];
    }
    int l=0,r=999999999,mid;
    while(l+1<r) (check(mid=l+r>>1)?r:l)=mid;
    cout<<l<<endl;
}