#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using db = double;
using str = string; // yay python!
using pi = pair<int,int>;
using pl = pair<ll,ll>;
using pd = pair<db,db>;
using vi = vector<int>;
using vb = vector<bool>;
using vl = vector<ll>;
using vd = vector<db>;
using vs = vector<str>;
using vpi = vector<pi>;
using vpl = vector<pl>;
using vpd = vector<pd>;
#define tcT template<class T
// ^ lol this makes everything look weird but I'll try it
tcT> using V = vector<T>;
tcT, size_t SZ> using AR = array<T,SZ>;
// pairs
#define mp make_pair
#define f first
#define s second
// vectors
#define sz(x) (int)(x).size()
#define all(x) begin(x), end(x)
#define rall(x) (x).rbegin(), (x).rend()
#define sor(x) sort(all(x))
#define rsz resize
#define ins insert
#define ft front()
#define bk back()
#define pf push_front
#define pb push_back
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
// loops
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a: x)
const int MOD = 1e9+7; // 998244353;
const int MX = 3e5+5;
const ll INF = 1e18; // not too close to LLONG_MAX
const ld PI = acos((ld)-1);
const int xd[4] = {1,0,-1,0}, yd[4] = {0,1,0,-1}; // for every grid problem!!
mt19937 rng((uint32_t)chrono::steady_clock::now().time_since_epoch().count());
// helper funcs
constexpr int pct(int x) { return __builtin_popcount(x); } // # of bits set
constexpr int bits(int x) { return 31-__builtin_clz(x); } // floor(log2(x))
ll cdiv(ll a, ll b) { return a/b+((a^b)>0&&a%b); } // divide a by b rounded up
ll fdiv(ll a, ll b) { return a/b-((a^b)<0&&a%b); } // divide a by b rounded down
tcT> bool ckmin(T& a, const T& b) {
return b < a ? a = b, 1 : 0; } // set a = min(a,b)
tcT> bool ckmax(T& a, const T& b) {
return a < b ? a = b, 1 : 0; }
#define tcTU tcT, class U
tcTU> T fstTrue(T lo, T hi, U f) {
hi ++; assert(lo <= hi); // assuming f is increasing
while (lo < hi) { // find first index such that f is true
T mid = lo+(hi-lo)/2;
f(mid) ? hi = mid : lo = mid+1;
}
return lo;
}
tcTU> T lstTrue(T lo, T hi, U f) {
lo --; assert(lo <= hi); // assuming f is decreasing
while (lo < hi) { // find first index such that f is true
T mid = lo+(hi-lo+1)/2;
f(mid) ? lo = mid : hi = mid-1;
}
return lo;
}
tcT> void remDup(vector<T>& v) { // sort and remove duplicates
sort(all(v)); v.erase(unique(all(v)),end(v)); }
tcTU> void erase(T& t, const U& u) { // don't erase
auto it = t.find(u); assert(it != end(t));
t.erase(u); } // element that doesn't exist from (multi)set
// INPUT
#define tcTUU tcT, class ...U
tcT> void re(complex<T>& c);
tcTU> void re(pair<T,U>& p);
tcT> void re(vector<T>& v);
tcT, size_t SZ> void re(AR<T,SZ>& a);
tcT> void re(T& x) { cin >> x; }
void re(db& d) { str t; re(t); d = stod(t); }
void re(ld& d) { str t; re(t); d = stold(t); }
tcTUU> void re(T& t, U&... u) { re(t); re(u...); }
tcT> void re(complex<T>& c) { T a,b; re(a,b); c = {a,b}; }
tcTU> void re(pair<T,U>& p) { re(p.f,p.s); }
tcT> void re(vector<T>& x) { trav(a,x) re(a); }
tcT, size_t SZ> void re(AR<T,SZ>& x) { trav(a,x) re(a); }
// TO_STRING
#define ts to_string
str ts(char c) { return str(1,c); }
str ts(const char* s) { return (str)s; }
str ts(str s) { return s; }
str ts(bool b) {
#ifdef LOCAL
return b ? "true" : "false";
#else
return ts((int)b);
#endif
}
tcT> str ts(complex<T> c) {
stringstream ss; ss << c; return ss.str(); }
str ts(vector<bool> v) {
str res = "{"; F0R(i,sz(v)) res += char('0'+v[i]);
res += "}"; return res; }
template<size_t SZ> str ts(bitset<SZ> b) {
str res = ""; F0R(i,SZ) res += char('0'+b[i]);
return res; }
tcTU> str ts(pair<T,U> p);
tcT> str ts(T v) { // containers with begin(), end()
#ifdef LOCAL
bool fst = 1; str res = "{";
for (const auto& x: v) {
if (!fst) res += ", ";
fst = 0; res += ts(x);
}
res += "}"; return res;
#else
bool fst = 1; str res = "";
for (const auto& x: v) {
if (!fst) res += " ";
fst = 0; res += ts(x);
}
return res;
#endif
}
tcTU> str ts(pair<T,U> p) {
#ifdef LOCAL
return "("+ts(p.f)+", "+ts(p.s)+")";
#else
return ts(p.f)+" "+ts(p.s);
#endif
}
// OUTPUT
tcT> void pr(T x) { cout << ts(x); }
tcTUU> void pr(const T& t, const U&... u) {
pr(t); pr(u...); }
void ps() { pr("\n"); } // print w/ spaces
tcTUU> void ps(const T& t, const U&... u) {
pr(t); if (sizeof...(u)) pr(" "); ps(u...); }
// DEBUG
void DBG() { cerr << "]" << endl; }
tcTUU> void DBG(const T& t, const U&... u) {
cerr << ts(t); if (sizeof...(u)) cerr << ", ";
DBG(u...); }
#ifdef LOCAL // compile with -DLOCAL, chk -> fake assert
#define dbg(...) cerr << "Line(" << __LINE__ << ") -> [" << #__VA_ARGS__ << "]: [", DBG(__VA_ARGS__)
#define chk(...) if (!(__VA_ARGS__)) cerr << "Line(" << __LINE__ << ") -> function(" \
<< __FUNCTION__ << ") -> CHK FAILED: (" << #__VA_ARGS__ << ")" << "\n", exit(0);
#else
#define dbg(...) 0
#define chk(...) 0
#endif
// FILE I/O
void setIn(str s) { freopen(s.c_str(),"r",stdin); }
void setOut(str s) { freopen(s.c_str(),"w",stdout); }
void unsyncIO() { cin.tie(0)->sync_with_stdio(0); }
void setIO(str s = "") {
unsyncIO();
// cin.exceptions(cin.failbit);
// throws exception when do smth illegal
// ex. try to read letter into int
if (sz(s)) { setIn(s+".in"), setOut(s+".out"); } // for USACO
}
int N,A,B,par[MX];
bool inPath[MX];
vi adj[MX];
void dfs(int x) {
trav(t,adj[x]) if (par[t] == 0) {
par[t] = x;
dfs(t);
}
}
int need(int x, int y) {
vi tri;
trav(t,adj[x]) if (!inPath[t] && t != y) tri.pb(need(t,x));
sort(rall(tri));
int ans = 0;
F0R(i,sz(tri)) ckmax(ans,i+1+tri[i]);
return ans;
}
int get_max(int res, vi ans, vi path) {
int cur = 0;
F0R(i,sz(ans)) {
if (ans[i]+cur > res) return i;
if (ans[i]+cur < res) {
cur ++;
continue;
}
int x = ans[i]; vi tri;
trav(t,adj[x]) if (!inPath[t]) tri.pb(need(t,x));
sort(rall(tri));
F0R(z,sz(tri)) tri[z] += 1+z;
R0F(z,sz(tri)-1) ckmax(tri[z],tri[z+1]);
int j = 0; while (j < sz(tri) && tri[0] == tri[j]) j ++, cur ++;
cur ++;
}
return sz(ans);
// dbg(ans,path); exit(0);
}
int main() {
setIO(); re(N,A,B); // dbg(N,A,B); exit(0);
F0R(i,N-1) {
int x,y; re(x,y);
adj[x].pb(y), adj[y].pb(x);
}
// dbg(N,A,B); exit(0);
par[A] = -1; dfs(A);
// FOR(i,1,N+1) dbg(i,par[i]);
// exit(0);
vi path = {B}; while (path.bk != A) path.pb(par[path.bk]);
// dbg(path);
trav(t,path) inPath[t] = 1;
vi ans; trav(t,path) ans.pb(need(t,-1));
int res = 0;
F0R(i,sz(ans)) ckmax(res,ans[i]+min(sz(ans)-1-i,i)+1);
int max_pref = get_max(res-1,ans,path);
reverse(all(path));
reverse(all(ans));
int max_pref_2 = get_max(res-1,ans,path);
if (max_pref+max_pref_2 >= sz(path)) ps(res-1);
else ps(res);
// vi pref, suf;
// {
// int mx = 0, cnt = 0;
// trav(t,ans) {
// ckmax(mx,t+cnt);
// pref.pb(mx);
// cnt ++;
// ckmax(mx,t+cnt);
// }
// }
// reverse(all(ans));
// {
// int mx = 0, cnt = 0;
// trav(t,ans) {
// ckmax(mx,t+cnt);
// suf.pb(mx);
// cnt ++;
// ckmax(mx,t+cnt);
// }
// }
// int res = MOD;
// F0R(i,sz(path)-1) ckmin(res,max(pref[i],suf[sz(path)-2-i]));
// ps(res);
// you should actually read the stuff at the bottom
}
/* stuff you should look for
* int overflow, array bounds
* special cases (n=1?)
* do smth instead of nothing and stay organized
* WRITE STUFF DOWN
* DON'T GET STUCK ON ONE APPROACH
*/
Compilation message
torrent.cpp: In function 'void setIn(str)':
torrent.cpp:182:28: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
182 | void setIn(str s) { freopen(s.c_str(),"r",stdin); }
| ~~~~~~~^~~~~~~~~~~~~~~~~~~~~
torrent.cpp: In function 'void setOut(str)':
torrent.cpp:183:29: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
183 | void setOut(str s) { freopen(s.c_str(),"w",stdout); }
| ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
| # |
결과 |
실행 시간 |
메모리 |
Grader output |
| 1 |
Correct |
6 ms |
7424 KB |
Output is correct |
| 2 |
Correct |
6 ms |
7424 KB |
Output is correct |
| 3 |
Correct |
5 ms |
7424 KB |
Output is correct |
| # |
결과 |
실행 시간 |
메모리 |
Grader output |
| 1 |
Correct |
152 ms |
20412 KB |
Output is correct |
| 2 |
Correct |
152 ms |
24344 KB |
Output is correct |
| 3 |
Correct |
126 ms |
23452 KB |
Output is correct |
| 4 |
Correct |
132 ms |
26140 KB |
Output is correct |
| 5 |
Correct |
152 ms |
24184 KB |
Output is correct |
| 6 |
Correct |
133 ms |
23728 KB |
Output is correct |
| 7 |
Correct |
123 ms |
25368 KB |
Output is correct |
