Submission #2115741

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Source Code Expand

#include <bits/stdc++.h>
#define ADD(a, b) a = (a + ll(b)) % mod
#define MUL(a, b) a = (a * ll(b)) % mod
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define rep(i, a, b) for(int i = int(a); i < int(b); i++)
#define rer(i, a, b) for(int i = int(a) - 1; i >= int(b); i--)
#define all(a) (a).begin(), (a).end()
#define sz(v) (int)(v).size()
#define pb push_back
#define sec second
#define fst first
#define debug(fmt, ...) Debug(__LINE__, ":", fmt, ##__VA_ARGS__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<int, pi> ppi;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> mat;
typedef complex<double> comp;
void Debug() {cout << '\n'; }
template<class FIRST, class... REST>void Debug(FIRST arg, REST... rest){
	cout<<arg<<" ";Debug(rest...);}
template<class T>ostream& operator<<(ostream& out,const vector<T>& v) {
	out<<"[";if(!v.empty()){rep(i,0,sz(v)-1)out<<v[i]<<", ";out<<v.back();}out<<"]";return out;}
template<class S, class T>ostream& operator<<(ostream& out,const pair<S, T>& v){
	out<<"("<<v.first<<", "<<v.second<<")";return out;}
const int MAX_N = 200010;
const int MAX_V = 100010;
const double eps = 1e-6;
const ll mod = 1000000007;
const int inf = 1 << 29;
const ll linf = 1LL << 60;
const double PI = 3.14159265358979323846;
///////////////////////////////////////////////////////////////////////////////////////////////////

namespace CD { //centroid decomposition

	struct edge { int to, length; };

	vector<edge> G[MAX_N];
	bool centroid[MAX_N];
	int subtree_size[MAX_N];

	void init(int n) {
		rep(i, 0, n) G[i].clear();
	}
	void add_edge(int a, int b, int c) {
		G[a].push_back((edge){b, c});
		G[b].push_back((edge){a, c});
	}

	int compute_subtree_size(int v, int p) {
		int c = 1;
		rep(i, 0, (int)G[v].size()) {
			int w = G[v][i].to;
			if(w == p || centroid[w]) continue;
			c += compute_subtree_size(G[v][i].to, v);
		}
		subtree_size[v] = c;
		return c;
	}

	pi search_centroid(int v, int p, int t) { 
		pi res = pi(inf, -1);
		int s = 1, m = 0;
		rep(i, 0, (int)G[v].size()) {
			int w = G[v][i].to;
			if(w == p || centroid[w]) continue;

			MIN(res, search_centroid(w, v, t));

			MAX(m, subtree_size[w]);
			s += subtree_size[w];
		}
		MAX(m, t - s);
		MIN(res, pi(m, v));
		return res;
	}


	void solve_subproblem(int v) {
		compute_subtree_size(v, -1);
		int cen = search_centroid(v, -1, subtree_size[v]).sec;
		centroid[cen] = true;

		set<pi> s;

		rep(i, 0, (int)G[cen].size()) {
			int n = G[cen][i].to;
			if(centroid[n]) continue;
			s.insert(pi(subtree_size[n], n));
		}

		while(sz(s) >= 2) {
			auto p1 = (*s.begin()); s.erase(p1);
			auto p2 = (*s.begin()); s.erase(p2);
			int a = p1.sec, b = p2.sec;
			int c;
			cout << "? " << a + 1 << " " << b + 1 << endl;
			cin >> c;
			if(c == a + 1) {
				solve_subproblem(a);
				return;
			}
			else if(c == b + 1){
				solve_subproblem(b);
				return;
			}
		}
		if(sz(s) == 1) {
			auto p1 = (*s.begin());
			int a = p1.sec, b = cen;
			int c;
			cout << "? " << a + 1 << " " << b + 1 << endl;
			cin >> c;
			if(c == a + 1) {
				solve_subproblem(a);
				return;
			}
		}
		cout << "! " << cen + 1 << endl;
		return;

		centroid[cen] = false;
	}
}

int N, Q;

void solve() {
	cin >> N >> Q;
	rep(i, 0, N - 1) {
		int a, b;
		cin >> a >> b;
		a--; b--;
		CD::add_edge(a, b, 1);
	}
	CD::solve_subproblem(0);
}

int main() {
#ifndef LOCAL
	// ios::sync_with_stdio(false);
 //    cin.tie(0);
#endif
    cout << fixed;
	cout.precision(20);
	srand((unsigned int)time(NULL));
#ifdef LOCAL
	//freopen("in.txt", "wt", stdout); //for tester
    // freopen("in.txt", "rt", stdin);
#endif	
	solve();
#ifdef LOCAL
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
#endif
	return 0;
}

Submission Info

Judge Result