#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(long long i=0;i<(long long)(n);i++)
#define REP(i,k,n) for(long long i=k;i<(long long)(n);i++)
#define pb emplace_back
#define lb(v,k) (lower_bound(all(v),(k))-v.begin())
#define ub(v,k) (upper_bound(all(v),(k))-v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T,vector<T>,greater<T>>
#define dame(a) {out(a);return 0;}
#define decimal cout<<fixed<<setprecision(15);
#define all(a) a.begin(),a.end()
#define rsort(a) {sort(all(a));reverse(all(a));}
#define dupli(a) {sort(all(a));a.erase(unique(all(a)),a.end());}
typedef long long ll;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> PP;
typedef tuple<ll,ll,ll,ll> PPP;
using vi=vector<ll>;
using vvi=vector<vi>;
using vvvi=vector<vvi>;
using vvvvi=vector<vvvi>;
using vp=vector<P>;
using vvp=vector<vp>;
using vb=vector<bool>;
using vvb=vector<vb>;
const ll inf=1001001001001001001;
const ll INF=1001001001;
const ll mod=1000000007;
const double eps=1e-10;
template<class T> bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<class T> bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<class T> void out(T a){cout<<a<<'\n';}
template<class T> void outp(T a){cout<<'('<<a.fi<<','<<a.se<<')'<<'\n';}
template<class T> void outvp(T v){rep(i,v.size())cout<<'('<<v[i].fi<<','<<v[i].se<<')';cout<<'\n';}
template<class T> void outvvp(T v){rep(i,v.size())outvp(v[i]);}
template<class T> void outv(T v){rep(i,v.size()){if(i)cout<<' ';cout<<v[i];}cout<<'\n';}
template<class T> void outvv(T v){rep(i,v.size())outv(v[i]);}
template<class T> bool isin(T x,T l,T r){return (l)<=(x)&&(x)<=(r);}
template<class T> void yesno(T b){if(b)out("yes");else out("no");}
template<class T> void YesNo(T b){if(b)out("Yes");else out("No");}
template<class T> void YESNO(T b){if(b)out("YES");else out("NO");}
template<class T> void outset(T s){auto itr=s.begin();while(itr!=s.end()){if(itr!=s.begin())cout<<' ';cout<<*itr;itr++;}cout<<'\n';}
void outs(ll a,ll b){if(a>=inf-100)out(b);else out(a);}
ll gcd(ll a,ll b){if(b==0)return a;return gcd(b,a%b);}
ll modpow(ll a,ll b){ll res=1;a%=mod;while(b){if(b&1)res=res*a%mod;a=a*a%mod;b>>=1;}return res;}
int main(){
cin.tie(0);ios_sync_with_stdio(false);
ll n,rt,st;cin>>n>>rt>>st;rt--;st--;
if(rt==st)dame(0);
vvi g(n);rep(i,n-1){
ll a,b;cin>>a>>b;a--;b--;
g[a].pb(b);g[b].pb(a);
}
vi dp(n);
vi par(n);
function<void(ll,ll)> dfs=[&](ll i,ll p){
par[i]=p;
vi srt;
for(ll x:g[i])if(x!=p){
dfs(x,i);
srt.pb(dp[x]);
}rsort(srt);
if(srt.size()>1)dp[i]=srt[1];
dp[i]+=srt.size();
};dfs(rt,-1);
ll ok=n*2,ng=-1;
// outv(dp);
while(ok-ng>1){
ll md=(ok+ng)/2;
ll cnt=0;
ll cur=st;
vi num,al;
while(cur!=rt){
al.pb(cur);cur=par[cur];
}reverse(all(al));
for(ll x:al)num.pb(g[x].size()-2);num.back()++;
vi rui=num;
rep(i,rui.size()-1)rui[i+1]+=rui[i];
bool able=true;
for(int i=al.size()-1;i>=0;i--){
ll tmp=0;vi srt;
for(ll x:g[al[i]])if(x!=par[al[i]]&&(i==al.size()-1||x!=al[i+1])){
if(cnt+rui[i]+dp[x]>md)tmp++;
}
cnt+=tmp;
if(cnt>al.size()-i||cnt>md)able=false;
}
if(able)ok=md;
else ng=md;
}
out(ok);
}
Compilation message
mousetrap.cpp: In function 'int main()':
mousetrap.cpp:52:16: error: 'ios_sync_with_stdio' was not declared in this scope
52 | cin.tie(0);ios_sync_with_stdio(false);
| ^~~~~~~~~~~~~~~~~~~
mousetrap.cpp:81:9: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
81 | for(ll x:al)num.pb(g[x].size()-2);num.back()++;
| ^~~
mousetrap.cpp:81:43: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'for'
81 | for(ll x:al)num.pb(g[x].size()-2);num.back()++;
| ^~~
mousetrap.cpp:87:51: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
87 | for(ll x:g[al[i]])if(x!=par[al[i]]&&(i==al.size()-1||x!=al[i+1])){
| ~^~~~~~~~~~~~~
mousetrap.cpp:91:19: warning: comparison of integer expressions of different signedness: 'll' {aka 'long long int'} and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
91 | if(cnt>al.size()-i||cnt>md)able=false;
| ~~~^~~~~~~~~~~~
