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先钦定树的重心处理出答案

若当前点是距离最大点对的lca(即在它们之间的路径上),这个点就是最优的

否则最优答案一定在最大点对的lca的子树中

每次都选树的重心来处理,最多需要处理\log n次,总复杂度\Theta(n \log n)

#include<bits/stdc++.h>
namespace ZDY{
    #pragma GCC optimize(3)
    #define il __inline__ __attribute__ ((always_inline))
    #define register
    #define ll long long
    #define ull unsigned long long
    #define db double
    #define sht short
    #define MB template <class T>il
    #define Fur(i,x,y) for(int i(x);i<=y;++i)
    #define Fdr(i,x,y) for(int i(x);i>=y;--i)
    #define fl(i,x) for(int i(head[x]),to;to=e[i].to,i;i=e[i].nxt)
    #define clr(x,y) memset(x,y,sizeof(x))
    #define cpy(x,y) memcpy(x,y,sizeof(x))
    #define fin(s) freopen(s".in","r",stdin)
    #define fout(s) freopen(s".out","w",stdout)
    #define fcin ios::sync_with_stdio(false)
    #define l2(n) ((int)(log2(n)))
    #define inf 2122219134
    MB T ABS(T x){return x>0?x:-x;}
    MB T MAX(T x,T y){return x>y?x:y;}
    MB T MIN(T x,T y){return x<y?x:y;}
    MB T GCD(T x,T y){return y?GCD(y,x%y):x;}
    MB void SWAP(T &x,T &y){T t=x;x=y;y=t;}
}using namespace ZDY;using namespace std;
namespace IO{const int str=1<<20;static char in_buf[str],*in_s,*in_t;bool __=0;il char gc(){return (in_s==in_t)&&(in_t=(in_s=in_buf)+fread(in_buf,1,str,stdin)),in_s==in_t?EOF:*in_s++;}il void in(string &ch){ch.clear();if(__)return;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF){__=1;return;}ch+=c;while((c=gc())!=EOF&&!isspace(c))ch+=c;if(c==EOF)__=1;}il void in(char &ch){if(__)return;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF)__=1;else ch=c;}il void in(char *ch){*ch='\0';if(__)return;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF){__=1;return;}*ch=c;ch++;while((c=gc())!=EOF&&!isspace(c))*ch=c,ch++;if(c==EOF)__=1;*ch='\0';}template<typename T>il void in(T &x){if(__)return;char c=gc();bool f=0;while(c!=EOF&&(c<'0'||c>'9'))f^=(c=='-'),c=gc();if(c==EOF){__=1;return;}x=0;while(c!=EOF&&'0'<=c&&c<='9')x=x*10+c-48,c=gc();if(c==EOF)__=1;if(f)x=-x;}template<typename T,typename ... arr>il void in(T &x,arr & ... y){in(x),in(y...);}const char ln='\n';static char out_buf[str],*out_s=out_buf,*out_t=out_buf+str;il void flush(){fwrite(out_buf,1,out_s-out_buf,stdout);out_s=out_buf;}il void pt(char c){(out_s==out_t)?(fwrite(out_s=out_buf,1,str,stdout),*out_s++=c):(*out_s++=c);}il void out(const char* s){while(*s)pt(*s++);}il void out(char* s){while(*s)pt(*s++);}il void out(char c){pt(c);}il void out(string s){for(int i=0;s[i];i++)pt(s[i]);}template<typename T>il void out(T x){if(!x){pt('0');return;}if(x<0)pt('-'),x=-x;char a[50],t=0;while(x)a[t++]=x%10,x/= 10;while(t--)pt(a[t]+'0');}template<typename T,typename ... arr>il void out(T x,arr & ... y){out(x),out(y...);}}using namespace IO;
#define N 100011
int n,m,cnt=0,head[N];
struct edge{
    int to,nxt,w;
}e[N<<1];
il void add(int x,int y,int w){
    e[++cnt].to=y;e[cnt].nxt=head[x];head[x]=cnt;e[cnt].w=w;
}
int siz[N],ans=inf,rt=0,mxs=inf,SZ,d[N],U[N],V[N],q[N],bl[N];
bool v[N];
void frt(int x,int f){
    siz[x]=1;
    int res=0;
    fl(i,x)if(!v[to]&&to!=f){
        frt(to,x);
        siz[x]+=siz[to];
        res=MAX(res,siz[to]);
    }
    res=MAX(res,SZ-siz[x]);
    if(res<mxs)rt=x,mxs=res;
}
void gd(int x,int f,int BL){
    bl[x]=BL;
    fl(i,x)if(to!=f)d[to]=d[x]+e[i].w,gd(to,x,BL);
}
il void op(){
    cout<<ans<<endl;
    exit(0);
}
void sol(int x){
    if(v[x])op();
    v[x]=1;d[x]=0;
    fl(i,x)d[to]=e[i].w,gd(to,x,to);
    int mx=0,len=0,las=0;
    Fur(i,1,m)
        if(d[U[i]]+d[V[i]]>mx)mx=d[U[i]]+d[V[i]],q[len=1]=i;
        else if(d[U[i]]+d[V[i]]==mx)q[++len]=i;
    ans=MIN(ans,mx);
    Fur(i,1,len){
        if(bl[U[q[i]]]!=bl[V[q[i]]])op();
        if(!las)las=bl[U[q[i]]];
        if(bl[U[q[i]]]!=las)op();
    }
    mxs=inf,SZ=siz[las];frt(las,0);sol(rt);
}
int main(){
    in(n,m);
    int x,y,w;
    Fur(i,2,n)in(x,y,w),add(x,y,w),add(y,x,w);
    Fur(i,1,m)in(U[i],V[i]);
    SZ=n,mxs=inf;
    frt(1,0);
    sol(rt);
    op();
}
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