运输计划

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
const int maxn=3e5+10;
struct edge{
    int next,to,dis;
}e[2*maxn],q[2*maxn];
struct length{
    int len,lca,u,v;
}len[maxn];
int head[maxn],cnt,headq[maxn],dis[maxn],maxlen,n,m,a[maxn],ans,s[maxn],num,ret,f[maxn];   
bool vis[maxn];
inline int readn()   
{
    int x=0;
    char ch=getchar();
    while(ch>‘9‘||ch<‘0‘)
        ch=getchar();
    while(ch>=‘0‘&&ch<=‘9‘)
    {
        x=(x<<3)+(x<<1)+(ch^‘0‘);
        ch=getchar();
    }
    return x;
}
inline void add_edge(int x,int y,int d)
{
    e[++cnt].next=head[x];
    e[cnt].to=y;
    e[cnt].dis=d;
    head[x]=cnt;
}
inline void add_que(int x,int y)
{
    q[++cnt].next=headq[x];
    q[cnt].to=y;
    headq[x]=cnt;
}
int find(int x)
{
    return f[x]==x?f[x]:f[x]=find(f[x]);    
}
void dfs(int u,int pre)
{
    for(int i=head[u];i;i=e[i].next)
    {
        int v=e[i].to;
        if(v==pre)
            continue;
        dfs(v,u);
        s[u]+=s[v];   
    }
    if(s[u]==num&&a[u]>ret)
        ret=a[u];  
}
inline bool check(int x)
{
    memset(s,0,sizeof(s));
    num=ret=0;
    for(int i=1;i<=m;i++)  
        if(len[i].len>x)
        {
            s[len[i].u]++;
            s[len[i].v]++;
            s[len[i].lca]-=2;
            num++;      
        }
    dfs(1,0);     
    if(maxlen-ret>x)    
        return 0;
    return 1;   
}
void tarjan(int u,int pre)     
{
    for(int i=head[u];i;i=e[i].next)
    {
        int v=e[i].to;
        if(v==pre)
            continue;
        dis[v]=dis[u]+e[i].dis;
        tarjan(v,u);
        a[v]=e[i].dis;
        int f1=find(v);
        int f2=find(u);
        if(f1!=f2)
            f[f1]=find(f2);
        vis[v]=1;
    }
    for(int i=headq[u];i;i=q[i].next)
        if(vis[q[i].to])
        {
            int p=(i+1)>>1;
            len[p].lca=find(q[i].to);
            len[p].len=dis[u]+dis[q[i].to]-2*dis[len[p].lca];
            maxlen=max(maxlen,len[p].len);
        }
}
int main()
{
    n=readn(),m=readn();
    for(int i=1;i<n;i++)
    {
        int ai=readn(),bi=readn(),ti=readn();
        add_edge(ai,bi,ti);    
        add_edge(bi,ai,ti);
    }
    for(int i=1;i<=n;i++)
        f[i]=i;
    cnt=0;
    for(int i=1;i<=m;i++)
    {
        int x=readn(),y=readn();        
        len[i].u=x;
        len[i].v=y;
        add_que(x,y);
        add_que(y,x);
    }
    tarjan(1,0);
    int l=0,r=maxlen,mid;
    while(l<=r)   
    {
        mid=(l+r)>>1;
        if(check(mid))
        {
            r=mid-1;
            ans=mid;  
        }
        else
            l=mid+1;     
    }
    printf("%d\n",ans);
    return 0;
}