Submission #1025065

#	Time	Username	Problem	Language	Result	Execution time	Memory
1025065		phoenix	The Xana coup (BOI21_xanadu)	C++17	100 / 100	42 ms	17004 KiB

xanadu

#include <bits/stdc++.h>

using namespace std;
using ll = long long;

const int N = 100100;
const int INF = 1e9;

int n;
vector<int> g[N];
int a[N];

int dp[N][2][2];

void dfs(int s, int p) {
    for (int to : g[s]) if (to != p) {
        dfs(to, s);
    }
    for (int x = 0; x <= 1; x++) {
        int f[2]{0, INF};
        for (int to : g[s]) if (to != p) {
            bool t = (x ^ a[to]);
            int g[2]{INF, INF};
            for (int xc = 0; xc <= 1; xc++) {
                for (int yc = 0; yc <= 1; yc++) {
                    if ((xc ^ yc) != (t)) continue;
                    for (int pr = 0; pr <= 1; pr++) {
                        g[(pr ^ xc)] = min(g[(pr ^ xc)], f[pr] + dp[to][xc][yc]);
                    }
                }
            }
            f[0] = g[0];
            f[1] = g[1];
        }
        dp[s][x][0] = f[0] + x;
        dp[s][x][1] = f[1] + x;
    }
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);

    cin >> n;
    for (int i = 1; i < n; 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, 1);
    int res = INF;
    for (int x = 0; x <= 1; x++) for (int y = 0; y <= 1; y++)
        if ((x ^ y) == a[1]) 
            res = min(res, dp[1][x][y]);
    
    if (res == INF) {
        cout << "impossible\n";
        return 0;    
    } else {
        cout << res << '\n';
        return 0;
    }
}

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