Submission #1124126

#	Time	Username	Problem	Language	Result	Execution time	Memory
1124126		mmdrzada	The Xana coup (BOI21_xanadu)	C++20	100 / 100	50 ms	24648 KiB

xanadu

#include <bits/stdc++.h>
 
using namespace std;

typedef vector<int> vi;
typedef vector<char> vc;
typedef pair<int, int> pii;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef vector<ll> vll;

#define sep ' '
#define F first
#define S second
#define fastIO   ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define pb push_back
#define kill(x) {cout << x << endl;return;}
#define sz(x) int(x.size())
#define SP(x) setprecision(x) 
#define mod(x) (1ll*x%M+M)%M
#define pq priority_queue
#define mid (l+r)/2
// #define mid2 (l+r+1)/2
#define pll pair<ll, ll>
#define REP(i, k) for (int i = 0 ; i < k ; i ++)
#define MP make_pair
#define us unordered_set

const int INF = 1e9+10;
const int N = 1e5+10;
int n;
vi adj[N];
bool s[N];
int dp[2][2][N];

void dfs(int v = 0, int p = -1) {
	// cout << v+1 << endl;
	if (adj[v].size() == 1 && adj[v][0] == p) {
		if (s[v]) {
			dp[1][0][v] = 1;
			dp[0][1][v] = 0;
		} else {
			dp[0][0][v] = 0;
			dp[1][1][v] = 1;
		}
		return;
	}
	bool flag0 = true, flag1 = true;
	for(int neigh: adj[v]) {
		if (neigh == p) continue;
		dfs(neigh, v);
		if (dp[0][0][neigh] == -1 && dp[1][0][neigh] == -1) {
			flag0 = false;
		}
		if (dp[0][1][neigh] == -1 && dp[1][1][neigh] == -1) {
			flag1 = false;
		}
	}
	if (flag0) {
		bool curr = s[v];
		vector<int> vec;
		int cost = 0;
		for(int neigh: adj[v]) {
			if (neigh == p) continue;
			if (dp[0][0][neigh] == -1) {
				cost += dp[1][0][neigh];
				curr = !curr;
			} else {
				cost += dp[0][0][neigh];
				if (dp[1][0][neigh] != -1) {
					vec.pb(dp[1][0][neigh] - dp[0][0][neigh]);
				}
			}
		}
		dp[0][curr][v] = (dp[0][curr][v] == -1 ? cost : min(dp[0][curr][v], cost));
		sort(vec.begin(), vec.end());
		for(int delta: vec) {
			cost += delta;
			curr = !curr;
			dp[0][curr][v] = (dp[0][curr][v] == -1 ? cost : min(dp[0][curr][v], cost));
		}
	}
	if (flag1) {
		bool curr = !s[v];
		vector<int> vec;
		int cost = 1; // we changed the v itself !
		for(int neigh: adj[v]) {
			if (neigh == p) continue;
			if (dp[0][1][neigh] == -1) {
				cost += dp[1][1][neigh];
				curr = !curr;
			} else {
				cost += dp[0][1][neigh];
				if (dp[1][1][neigh] != -1) {
					vec.pb(dp[1][1][neigh] - dp[0][1][neigh]);
				}
			}
		}
		dp[1][curr][v] = (dp[1][curr][v] == -1 ? cost : min(dp[1][curr][v], cost));
		sort(vec.begin(), vec.end());
		for(int delta: vec) {
			cost += delta;
			curr = !curr;
			dp[1][curr][v] = (dp[1][curr][v] == -1 ? cost : min(dp[1][curr][v], cost));
		}
	}

}

void solve() {
	memset(dp, -1, sizeof dp);
	cin >> n;
	for(int i = 0 ; i < n-1 ; i ++) {
		int u, v; cin >> u >> v; u--, v--;
		adj[u].pb(v), adj[v].pb(u);
	}
	REP(i, n) cin >> s[i];
	
	dfs();
	int ans = n+1;
	if (dp[0][0][0] != -1) ans = min(ans, dp[0][0][0]);
	if (dp[1][0][0] != -1) ans = min(ans, dp[1][0][0]);
	if (ans <= n) cout << ans << endl;
	else cout << "impossible" << endl;
}

// check MAXN
int32_t main() {
	fastIO;
	
	solve();
	
	return 0;
}

• Subtask 1: 5 / 5
• Subtask 2: 15 / 15
• Subtask 3: 10 / 10
• Subtask 4: 40 / 40
• Subtask 5: 30 / 30