Submission #401963

#	Time	Username	Problem	Language	Result	Execution time	Memory
401963		blue	The Xana coup (BOI21_xanadu)	C++17	100 / 100	178 ms	22716 KiB

xanadu

#include <iostream>
#include <vector>
using namespace std;

/*
dp[u][flag1][flag2] = minimum toggles from descendants of u only required such that:
                        state of u is flag1
                        state of all direct children of u is flag2
                        state of all other descendants is 0.

              if this is impossible, 1e9;
*/

vector<int> edge[100001];
int sv[100001];

vector<int> p(100001, 0);
vector<int> children[100001];

int dp[100001][2][2];


void dfs1(int u)
{
    for(int v: edge[u])
    {
        if(p[u] == v) continue;

        p[v] = u;
        children[u].push_back(v);

        dfs1(v);
    }
}

void dfs2(int u)
{
    for(int v: children[u])
        dfs2(v);

    dp[u][ sv[u] ][0] = dp[u][ sv[u] ][1] = 0;

    //dp_curr[u][flag1][flag2] = state of dp assuming it has only previously visited children

    for(int v: children[u])
    {
        int X[2][2];

        for(int f = 0; f <= 1; f++)
            for(int g = 0; g <= 1; g++)
                X[f][g] = min(dp[u][f][g] + dp[v][g][0], dp[u][!f][g] + dp[v][!g][1] + 1);

        for(int f = 0; f <= 1; f++)
            for(int g = 0; g <= 1; g++)
                dp[u][f][g] = X[f][g];
    }

    // cout << u << ' ' << dp[u][0][0] << ' ' << dp[u][0][1] << ' ' << dp[u][1][0] << ' ' << dp[u][1][1] << '\n';
}

int main()
{
    int N;
    cin >> N;

    for(int i = 1; i <= N-1; i++)
    {
        int a, b;
        cin >> a >> b;
        edge[a].push_back(b);
        edge[b].push_back(a);
    }

    edge[1].push_back(0);
    edge[0].push_back(1);

    dfs1(0);

    for(int i = 0; i <= N; i++)
    {
        for(int f = 0; f <= 1; f++)
            for(int g = 0; g <= 1; g++)
                dp[i][f][g] = N+1;
    }

    for(int i = 1; i <= N; i++)
        cin >> sv[i];

    dp[0][0][0] = dp[0][0][1] = dp[0][1][0] = dp[0][1][1] = 0;
    dfs2(0);

    int res = min(dp[0][0][0], dp[0][1][0]);
    if(res >= N+1) 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