CSES - Shared codeLink to this code:
https://cses.fi/paste/dbc9a4a192220ac7372878/
/*
=============================================================================
/\ \ / /\ |\ /| |===========
/ \ \ / / \ | \ / | |
/ \ \ / / \ | \ / | |
/ \ | / \ | \ / | |===========
/--------\ | /--------\ | \ / | |
/ \ | / \ | \ / | |
/ \ | / \ | | | |===========
A A
AAAAAAAA AAA AAAA A A A A A A
A A A A A A A A A A A A A
A A AA A A A A A A A A A
A A A AA A A A A A A A
A AAA A A AAA AAA AAAAA
=============================================================================
*/
#include<bits/stdc++.h>
#define pb push_back
#define pf push_front
#define mp make_pair
#define F first
#define S second
#define pq priority_queue
#define ll long long
#define pii pair<int,int>
#define endl '\n'
#define Orz ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define MAXN 200001
using namespace std;
//YeedragOrz
//8e7Orz
//jassOrz
//FHvirusOrz
//samOrz
//yungyaoOrz
//littlecubeOrz
//2qbingxuanOrz
//FoodSheepOrz
//ck_platypusOrz
//koyingOrz
//foxyyOrz
//peiganOrz
//jikuaiOrz
vector<int> gra[MAXN];
int dep[MAXN],spa[MAXN][20];
void dfs(int pos,int pre){
for(auto i:gra[pos]){
if(i!=pre){
dep[i]=dep[pos]+1;
spa[i][0]=pos;
dfs(i,pos);
}
}
}
int lca(int a,int b){
if(dep[a]>dep[b])swap(a,b);
int gap=dep[b]-dep[a];
for(int i=0;i<=19;i++){
if((1<<i)&gap)b=spa[b][i];
}
if(a==b)return a;
for(int i=19;i>=0;i--){
if(spa[a][i]!=spa[b][i]){
a=spa[a][i];
b=spa[b][i];
}
}
return spa[a][0];
}
int main(){
int n,q;cin>>n>>q;
for(int i=1;i<n;i++){
int a,b;cin>>a>>b;
gra[a].pb(b);
gra[b].pb(a);
}
dep[1]=0;
spa[1][0]=1;
dfs(1,-1);
for(int i=1;i<=19;i++){
for(int j=1;j<=n;j++)spa[j][i]=spa[spa[j][i-1]][i-1];
}
while(q--){
int a,b;cin>>a>>b;
cout<<dep[a]+dep[b]-dep[lca(a,b)]*2<<endl;
}
return 0;
}