This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <iostream>
#include <utility>
#include <vector>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long ll;
typedef pair <int, int> pii;
#define x first
#define y second
#define mp make_pair
int n, tx, ty, dth[300005];
vector <pii> ms[300005];
bool bio[300005];
vector <int> naputu;
pii par[300005];
const ll inf = (ll)1e15;
bool cmp(ll x, ll y){
return x > y;
}
void dfs1(int x, int p){
for(vector <pii>::iterator i = ms[x].begin() ; i != ms[x].end() ; ++i){
pii o = *i;
if(o.x == p || bio[o.y])
continue;
par[o.x] = mp(x, o.y);
dth[o.x] = dth[x] + 1;
dfs1(o.x, x);
}
}
ll dfs2(int x, int p){
ll ret = 0;
vector <ll> v;
for(vector <pii>::iterator i = ms[x].begin() ; i != ms[x].end() ; ++i){
pii o = *i;
if(o.x == p || bio[o.y])
continue;
v.push_back(dfs2(o.x, x));
}
sort(v.begin(), v.end(), cmp);
for(int i = 0 ; i < (int)v.size() ; ++i){
ret = max(ret, v[i] + i + 1);
}
return ret;
}
void odigore(int &x, int k, vector <int> &v){
for(int i = 0 ; i < k ; ++i){
v.push_back(par[x].y);
x = par[x].x;
}
}
void nadiput(int x, int y){
vector <int> prvi, drugi;
if(dth[x] > dth[y]){
odigore(x, dth[x] - dth[y], prvi);
}
else if(dth[y] > dth[x]){
odigore(y, dth[y] - dth[x], drugi);
}
while(x != y){
prvi.push_back(par[x].y);
x = par[x].x;
drugi.push_back(par[y].y);
y = par[y].x;
}
for(int i = 0 ; i < (int)prvi.size() ; ++i){
naputu.push_back(prvi[i]);
}
for(int i = (int)drugi.size() - 1 ; i >= 0 ; --i){
naputu.push_back(drugi[i]);
}
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> n >> tx >> ty;
for(int i = 0 ; i < n - 1 ; ++i){
int x, y;
cin >> x >> y;
ms[x].push_back(mp(y, i));
ms[y].push_back(mp(x, i));
}
dfs1(1, -1);
nadiput(tx, ty);
int l = 0, r = (int)naputu.size() - 1;
ll sol = inf;
while(l < r){
int mid = (l + r) / 2;
bio[naputu[mid]] = 1;
ll x = dfs2(tx, -1);
ll y = dfs2(ty, -1);
if(x < y){
l = mid + 1;
}
else{
r = mid;
}
sol = min(sol, max(x, y));
bio[naputu[mid]] = 0;
}
bio[l] = 1;
sol = min(sol, max(dfs2(tx, -1), dfs2(ty, -1)));
cout << sol << endl;
return 0;
}
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