Submission #1188830

#	Time	Username	Problem	Language	Result	Execution time	Memory
1188830		12345678	Star Trek (CEOI20_startrek)	C++20	0 / 100	1095 ms	2632 KiB

startrek

#include <bits/stdc++.h>

using namespace std;

#define ll long long

const int nx=1e5+5, mod=1e9+7;

ll n, k, u, v, dp[nx], dpr[nx], lvl[nx], force[nx], lcnt, res;
vector<int> d[nx];

void dfs(int u, int p)
{
    lvl[u]=lvl[p]+1;
    for (auto v:d[u]) if (v!=p) dfs(v, u), dp[u]|=!dp[v];
}

void fillforce(int u, int p)
{
    int cnt=0;
    for (auto v:d[u]) if (v!=p) cnt++;
    if (cnt==1) for (auto v:d[u]) if (v!=p) force[v]=1, fillforce(v, u);
    if ((lvl[u]%2)&&!dp[u]) for (auto v:d[u]) if (v!=p) force[v]=1, fillforce(v, u);
    if ((lvl[u]%2)&&dp[u])
    {
        int cnt=0;
        for (auto v:d[u]) if (v!=p&&!dp[v]) cnt++;
        if (cnt==1) for (auto v:d[u]) if (v!=p&&!dp[v]) force[v]=1, fillforce(v, u);
    }
}

void dfs2(int u, int p)
{
    lcnt+=!(dp[u]|dpr[u]);
    //cout<<"debug "<<u<<' '<<dp[u]<<' '<<dpr[u]<<'\n';
    int cnt=0, tot=0;
    for (auto v:d[u]) if (v!=p) tot++;
    for (auto v:d[u]) if (v!=p&&dp[v]) cnt++;
    //cout<<"tot "<<cnt<<' '<<tot<<'\n';
    for (auto v:d[u])
    {
        if (v==p) continue;
        if (!dpr[u]&&(cnt-dp[v])==tot-1) dpr[v]=1;
        dfs2(v, u);
    }
}

int main()
{
    cin.tie(NULL)->sync_with_stdio(false);
    cin>>n>>k;
    for (int i=1; i<n; i++) cin>>u>>v, d[u].push_back(v), d[v].push_back(u);
    dfs(1, 1);
    force[1]=1;
    fillforce(1, 1);
    //for (int i=1; i<=n;i++) cout<<"f "<<i<<' '<<force[i]<<'\n';
    dfs2(1, 1);
    //cout<<"lcnt "<<lcnt<<'\n';
    if (dp[1])
    {
        for (int i=1 ;i<=n; i++)
        {
            if (!force[i]) res=(res+n)%mod;
            else if (lvl[i]%2) res=(res+n)%mod;
            else res=(res+(n-lcnt))%mod;
        }
    }
    else
    {
        for (int i=1; i<=n ;i++)
        {
            if (!force[i]) continue;
            if (lvl[i]%2) res=(res+lcnt)%mod;
        }
    }
    cout<<res;
}

/*
5 1
1 2
1 3
3 4
3 5
*/

• Subtask 1: 0 / 0
• Subtask 2: 0 / 7
• Subtask 3: 0 / 8
• Subtask 4: 0 / 15
• Subtask 5: 0 / 15
• Subtask 6: 0 / 20
• Subtask 7: 0 / 20
• Subtask 8: 0 / 15