蒟蒻求教！！！

#include<bits/stdc++.h>
using namespace std;
int n,m;
int head[1<<20],length[1<<20],depth[1<<20],fa[1<<20][23],sum[1<<20];
int dis[1<<20];
int cnt,num,ret,maxlen;
struct dataB{
    int x,y,length;
}edge[1<<20];
struct dataA{
    int to,next,value;
}e[1<<20];
inline void insert(int u,int v,int w)
{
    e[++cnt].to=v;
    e[cnt].next=head[u];
    e[cnt].value=w;
    head[u]=cnt;
}
void dfs_depth(int x)
{
    for(int i=head[x];i;i=e[i].next)
    {
        int v=e[i].to;
        if(!depth[v])
        {
            length[v]=e[i].value;
        //  maxlen=max(maxlen,length[v]);
            depth[v]=depth[x]+1;
            dis[v]=dis[x]+e[i].value;
            fa[v][0]=x;
            dfs_depth(v);
        }
    }
    return ;    
}
void differ(int x)
{
    for(int i=head[x];i;i=e[i].next)
    {
        int v=e[i].to;
        if(v!=fa[x][0])
        {
            differ(v);
            sum[x]+=sum[v];
        }
    }
    if(sum[x]==num&&length[x]>ret)
    {
        ret=length[x];
    }
    return ;
}
int lca(int x,int y)
{
    if(depth[x]<depth[y])swap(x,y);
    for(int i=20;i>=0;i--)
    {
        if(depth[fa[x][i]]>=depth[y])
        {
            x=fa[x][i];
        }
    }
    if(x==y)return x;
    for(int i=20;i>=0;i--)
    {
        if(fa[x][i]!=fa[y][i])
        x=fa[x][i],y=fa[y][i];
    }
    return fa[x][0];
}
bool check_ans(int x)
{
    memset(sum,0,sizeof(sum));
    num=ret=0;
    for(int i=1;i<=m;i++)
    {
        if(edge[i].length>x)
        {
            num++;
            sum[edge[i].x]++;
            sum[edge[i].y]++;
            sum[lca(edge[i].x,edge[i].y)]-=2;
            cout<<"!"<<lca(edge[i].x,edge[i].y)<<endl;
        }
    }
    differ(1);
    cout<<ret<<endl;
    if(maxlen-ret>x)
    return false;
    return true;
}
int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1;i<n;i++)
    {
        int u,v,w;
        scanf("%d%d%d",&u,&v,&w);
        insert(u,v,w);
        insert(v,u,w);
    }
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d",&edge[i].x,&edge[i].y);
    }
    depth[1]=1;
    dfs_depth(1);
    for(int i=1;i<=20;i++)
    for(int j=1;j<=n;j++)
    {
        fa[i][j]=fa[fa[i][j-1]][j-1];
    }
    for(int i=1;i<=m;i++)
    {
        edge[i].length=dis[edge[i].x]+dis[edge[i].y]-2*dis[lca(edge[i].x,edge[i].y)];
        maxlen=max(maxlen,edge[i].length);
    }
    int left=0,right=maxlen;
    while(left<right)
    {
        int mid=(left+right)>>1;
        if(check_ans(mid))right=mid;
        else
        left=mid+1;
    }
    printf("%d\n",left);
    return 0;
}