Submission #715329

#	Time	Username	Problem	Language	Result	Execution time	Memory
715329		stevancv	The Xana coup (BOI21_xanadu)	C++14	100 / 100	86 ms	16920 KiB

xanadu

#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define sp ' '
#define en '\n'
#define smin(a, b) a = min(a, b)
#define smax(a, b) a = max(a, b)
using namespace std;
const int N = 1e5 + 2;
const int inf = 1e9;
int dp[N][2][2], a[N];
vector<int> g[N];
void Dfs(int s, int e) {
    for (int j = 0; j < 4; j++) dp[s][j / 2][j % 2] = inf;
    if (g[s].size() == 1 && g[s][0] == e) {
        dp[s][a[s]][0] = 0;
        dp[s][1 - a[s]][1] = 1;
        return;
    }
    for (int u : g[s]) if (u != e) Dfs(u, s);
    int cnt, sum, mn, moze;
    cnt = sum = 0;
    mn = inf;
    moze = 1;
    for (int u : g[s]) {
        if (u == e) continue;
        sum += min(dp[u][0][0], dp[u][0][1]);
        if (min(dp[u][0][0], dp[u][0][1]) == inf) moze = 0;
        smin(mn, abs(dp[u][0][0] - dp[u][0][1]));
        if (dp[u][0][0] > dp[u][0][1]) cnt += 1;
    }
    if (moze == 1) {
        dp[s][0][0] = sum;
        if (mn <= 1000000) dp[s][1][0] = sum + mn;
        if ((cnt + a[s]) % 2 == 1) swap(dp[s][0][0], dp[s][1][0]);
    }
    cnt = sum = 0;
    mn = inf;
    moze = 1;
    for (int u : g[s]) {
        if (u == e) continue;
        sum += min(dp[u][1][0], dp[u][1][1]);
        if (min(dp[u][1][0], dp[u][1][1]) == inf) moze = 0;
        smin(mn, abs(dp[u][1][0] - dp[u][1][1]));
        if (dp[u][1][0] > dp[u][1][1]) cnt += 1;
    }
    if (moze == 1) {
        dp[s][0][1] = sum + 1;
        if (mn <= 1000000) dp[s][1][1] = sum + mn + 1;
        if ((cnt + a[s]) % 2 == 0) swap(dp[s][0][1], dp[s][1][1]);
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n; cin >> n;
    for (int i = 0; i < n - 1; i++) {
        int u, v;
        cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    for (int i = 1; i <= n; i++) cin >> a[i];
    Dfs(1, 0);
    int ans = min(dp[1][0][0], dp[1][0][1]);
    if (ans == inf) cout << "impossible" << en;
    else cout << ans << en;
    return 0;
}

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