Submission #417822

#	Time	Username	Problem	Language	Result	Execution time	Memory
417822		Kevin_Zhang_TW	The Xana coup (BOI21_xanadu)	C++17	100 / 100	136 ms	21584 KiB

xanadu

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define pb emplace_back
#define AI(i) begin(i), end(i)
template<class T> bool chmin(T &a, T b) { return b < a && (a = b, true); }
template<class T> bool chmax(T &a, T b) { return a < b && (a = b, true); }
#ifdef KEV
#define DE(args...) kout("[ " + string(#args) + " ] = ", args)
void kout() { cerr << endl; }
template<class T, class ...U> void kout(T a, U ...b) { cerr << a << ' ', kout(b...); }
template<class T> void debug(T l, T r) { while (l != r) cerr << *l << " \n"[next(l)==r], ++l; }
#else
#define DE(...) 0
#define debug(...) 0
#endif

const int MAX_N = 300010, inf = MAX_N;

int dp[MAX_N][2][2], n, state[MAX_N];
vector<int> edge[MAX_N];

void dfs(int x, int lst) {
	for (int u : edge[x]) if (u != lst)
		dfs(u, x);

	// consider when I don't touch the x'th switch
	{
		int A[2]{inf, inf}, B[2];
		A[ state[x] ] = 0;
		for (int u : edge[x]) if (u != lst) {
			B[0] = B[1] = inf;

			for (int j = 0;j < 2;++j) {
				// consider I don't touch both x, u
				chmin(B[j], A[j] + dp[u][0][0]);
				// consider I touch just u, not x
				chmin(B[j^1], A[j] + dp[u][1][0]);
			}
			swap(A, B);
		}
		dp[x][0][0] = A[0];
		dp[x][0][1] = A[1];
	}
	// consider when I touch the x'th switch
	{
		int A[2]{inf, inf}, B[2];
		A[ state[x]^1 ] = 1;
		for (int u : edge[x]) if (u != lst) {
			B[0] = B[1] = inf;

			for (int j = 0;j < 2;++j) {
				// consider I touch just x, not u
				chmin(B[j], A[j] + dp[u][0][1]);
				// consider I touch both x, u
				chmin(B[j^1], A[j] + dp[u][1][1]);
			}
			swap(A, B);
		}
		dp[x][1][0] = A[0];
		dp[x][1][1] = A[1];
	}
}

int32_t main() {
	ios_base::sync_with_stdio(0), cin.tie(0);
	cin >> n;
	for (int a, b, i = 1;i < n;++i) {
		cin >> a >> b;
		edge[a].pb(b), edge[b].pb(a);
	}
	for (int i = 1;i <= n;++i) cin >> state[i];

	dfs(1, 1);

	int res = min(dp[1][0][0], dp[1][1][0]);
	if (res == inf) {
		cout << "impossible\n";
	} else cout << res << '\n';

}

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