先来LCA的：
1 #include<iostream> 2 #include<cstdio> 3 #include<cmath> 4 #include<algorithm> 5 using namespace std; 6 const int maxn=200000+10; 7 struct Tedge{int x,y,next;}adj[maxn*2];int fch[maxn],ms=0; 8 void AddEdge(int u,int v){ 9 adj[++ms]=(Tedge){u,v,fch[u]};fch[u]=ms; adj[++ms]=(Tedge){v,u,fch[v]};fch[v]=ms; return; 10 } 11 int s[maxn],top[maxn],fa[maxn],son[maxn],dep[maxn],n,m; 12 void First_DFS(int x){ 13 dep[x]=dep[fa[x]]+1;s[x]=1; 14 for(int i=fch[x];i;i=adj[i].next){ 15 int v=adj[i].y;if(v==fa[x]) continue; 16 fa[v]=x;First_DFS(v); 17 if(s[son[x]]<s[v]) son[x]=v;s[x]+=s[v]; 18 } return ; 19 } 20 void Second_DFS(int x,int tp){ 21 top[x]=tp; 22 if(son[x]) Second_DFS(son[x],tp); 23 for(int i=fch[x];i;i=adj[i].next){ 24 int v=adj[i].y;if(v==fa[x]||v==son[x]) continue; 25 Second_DFS(v,v); 26 } return ; 27 } 28 void LCA(int& a, int& b){ 29 int f1=top[a],f2=top[b]; 30 while(f1!=f2){ 31 if(dep[f1]<dep[f2]) swap(a,b),swap(f1,f2); 32 a=fa[f1]; f1=top[a]; 33 } return ; 34 } 35 int dist(int a,int b){ 36 int ans=dep[a]+dep[b];LCA(a,b);return ans-min(dep[a],dep[b])*2; 37 } 38 inline int read(){ 39 int x=0,sig=1;char ch=getchar(); 40 while(!isdigit(ch)){if(ch=='-') sig=-1;ch=getchar();} 41 while(isdigit(ch)) x=10*x+ch-'0',ch=getchar(); 42 return x*=sig; 43 } 44 inline void write(int x){ 45 if(x==0){putchar('0');return;} if(x<0) putchar('-'),x=-x; 46 int len=0,buf[15]; while(x) buf[len++]=x%10,x/=10; 47 for(int i=len-1;i>=0;i--) putchar(buf[i]+'0');return; 48 } 49 void init(){ 50 n=read();m=read();for(int i=2;i<=n;i++) AddEdge(read(),read()); 51 First_DFS(1);Second_DFS(1,1);return; 52 } 53 void work(){ 54 int a,b,c,d[3]; 55 while(m--){ 56 a=read();b=read();c=read(); 57 d[0]=dist(a,b);d[1]=dist(b,c);d[2]=dist(a,c); 58 sort(d,d+3);write(d[1]);putchar(' '); 59 } return; 60 } 61 void print(){ 62 return; 63 } 64 int main(){ 65 init(); work(); print(); return 0; 66 }