Submission #532371

#	Time	Username	Problem	Language	Result	Execution time	Memory
532371		4fecta	The Xana coup (BOI21_xanadu)	C++17	100 / 100	68 ms	20420 KiB

xanadu

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define int ll
#define ld long double
#define pii pair<int, int>
#define f first
#define s second
#define boost() cin.tie(0), cin.sync_with_stdio(0)

const int MN = 100005;

int n, u, v, p[MN], dp[MN][2][2]; //node, our state, do we flip parent
vector<int> a[MN];

void dfs(int cur, int par) {
    int sum = 0, flip = 0, mn = 0x3f3f3f3f; //dont touch cur
    for (int nxt : a[cur]) {
        if (nxt == par) continue;
        dfs(nxt, cur);
        if (dp[nxt][0][0] < dp[nxt][0][1]) {
            sum += dp[nxt][0][0];
            mn = min(mn, dp[nxt][0][1] - dp[nxt][0][0]);
        } else {
            sum += dp[nxt][0][1];
            flip ^= 1;
            mn = min(mn, dp[nxt][0][0] - dp[nxt][0][1]);
        }
    }
    dp[cur][p[cur] ^ flip][0] = sum;
    dp[cur][p[cur] ^ flip ^ 1][0] = sum + mn;
    sum = 0, flip = 0, mn = 0x3f3f3f3f; //flip cur
    for (int nxt : a[cur]) {
        if (nxt == par) continue;
        if (dp[nxt][1][0] < dp[nxt][1][1]) {
            sum += dp[nxt][1][0];
            mn = min(mn, dp[nxt][1][1] - dp[nxt][1][0]);
        } else {
            sum += dp[nxt][1][1];
            flip ^= 1;
            mn = min(mn, dp[nxt][1][0] - dp[nxt][1][1]);
        }
    }
    dp[cur][p[cur] ^ flip ^ 1][1] = sum + 1;
    dp[cur][p[cur] ^ flip][1] = sum + mn + 1;
}

int32_t main() {
    boost();

    memset(dp, 0x3f, sizeof(dp));
    cin >> n;
    for (int i = 1; i < n; i++) {
        cin >> u >> v;
        a[u].push_back(v);
        a[v].push_back(u);
    }
    for (int i = 1; i <= n; i++) cin >> p[i];
    dfs(1, 0);
    int ans = min(dp[1][0][0], dp[1][0][1]);
    if (ans > n) printf("impossible\n");
    else printf("%lld\n", ans);

    return 0;
}

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