树链剖分

先来LCA的：

#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
const int maxn=200000+10;
struct Tedge{int x,y,next;}adj[maxn*2];int fch[maxn],ms=0;
void AddEdge(int u,int v){
    adj[++ms]=(Tedge){u,v,fch[u]};fch[u]=ms; adj[++ms]=(Tedge){v,u,fch[v]};fch[v]=ms; return;
}
int s[maxn],top[maxn],fa[maxn],son[maxn],dep[maxn],n,m;
void First_DFS(int x){
    dep[x]=dep[fa[x]]+1;s[x]=1;
    for(int i=fch[x];i;i=adj[i].next){
        int v=adj[i].y;if(v==fa[x]) continue;
        fa[v]=x;First_DFS(v);
        if(s[son[x]]<s[v]) son[x]=v;s[x]+=s[v];
    } return ;
}
void Second_DFS(int x,int tp){
    top[x]=tp;
    if(son[x]) Second_DFS(son[x],tp);
    for(int i=fch[x];i;i=adj[i].next){
        int v=adj[i].y;if(v==fa[x]||v==son[x]) continue;
        Second_DFS(v,v);
    } return ;
}
void LCA(int& a, int& b){
    int f1=top[a],f2=top[b];
    while(f1!=f2){
        if(dep[f1]<dep[f2]) swap(a,b),swap(f1,f2);
        a=fa[f1]; f1=top[a];
    } return ;
}
int dist(int a,int b){
    int ans=dep[a]+dep[b];LCA(a,b);return ans-min(dep[a],dep[b])*2;
}
inline int read(){
    int x=0,sig=1;char ch=getchar();
    while(!isdigit(ch)){if(ch=='-') sig=-1;ch=getchar();}
    while(isdigit(ch)) x=10*x+ch-'0',ch=getchar();
    return x*=sig;
}
inline void write(int x){
    if(x==0){putchar('0');return;} if(x<0) putchar('-'),x=-x;
    int len=0,buf[15]; while(x) buf[len++]=x%10,x/=10;
    for(int i=len-1;i>=0;i--) putchar(buf[i]+'0');return;
}
void init(){
    n=read();m=read();for(int i=2;i<=n;i++) AddEdge(read(),read());
    First_DFS(1);Second_DFS(1,1);return;
}
void work(){
    int a,b,c,d[3];
    while(m--){
        a=read();b=read();c=read();
        d[0]=dist(a,b);d[1]=dist(b,c);d[2]=dist(a,c);
        sort(d,d+3);write(d[1]);putchar('
');
    } return;
}
void print(){
    return;
}
int main(){
    init(); work(); print(); return 0;
}