Submission #667610

#	Time	Username	Problem	Language	Result	Execution time	Memory
667610		mychecksedad	Star Trek (CEOI20_startrek)	C++17	7 / 100	39 ms	23824 KiB

startrek

/* Author : Mychecksdead */
#include<bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
typedef long long int ll;
typedef long double ld;
#define MOD (1000000000+7)
#define MOD1 (998244353)
#define PI 3.1415926535
#define pb push_back
#define setp() cout << setprecision(15)
#define all(x) x.begin(), x.end()
#define debug(x) cerr << #x << " is " << x << '\n';
const int N = 1e6+100, M = 1e5+10, F = 2147483646, K = 20;

ll po(ll a, ll b){
    ll res = 1;
    while(b){
        if(b&1) (res *= a) %= MOD;
        (a *= a) %= MOD;
        b >>= 1;
    }
    return res;
}

ll n, d, dp[N][2], portal;
bool X = 0;
vector<int> g[N];

void dfs(int v, int p){
    dp[v][0] = dp[v][1] = 0;
    if(portal == v){
        dp[v][0] = X, dp[v][1] = n - X;
    }else if(g[v].size() == 1 && v != 1){
        dp[v][1] = max(dp[v][1], n);
        return;
    }
    for(int u: g[v]){
        if(u != p){
            dfs(u, v);
            dp[v][0] = max(dp[v][0], dp[u][1]);
            dp[v][1] = max(dp[v][1], dp[u][0]);
        }
    }
}

void pre(int v, int p, int root){
    if(g[v].size() == 1 && v != root){
        dp[v][0] = 0;
        dp[v][1] = 1;
        return;
    }
    dp[v][0] = dp[v][1] = 0;
    for(int u: g[v]){
        if(u != p){
            pre(u, v, root);
            dp[v][0] = max(dp[v][0], dp[u][1]);
            dp[v][1] = max(dp[v][1], dp[u][0]);
        }
    }
}

void solve(){
    cin >> n >> d;
    for(int i = 0 ; i < n - 1; ++i){
        int u, v; cin >> u >> v;
        g[u].pb(v);
        g[v].pb(u);
    }
    if(n == 2){
        cout << po(4, d); return;
    }
    ll ans = 0;

    for(int i = 1; i <= n; ++i){
        pre(i, 0, i);
        X += dp[i][1];
    }
    for(int i = 1; i <= n; ++i){
        portal = i;
        dfs(1, 0);    
        // for(int j = 1; j <= n; ++j) cout << dp[j][0] << ' ' << dp[j][1] << '.';
        // cout << '\n';
        (ans += dp[1][0]) %= MOD;
    }
    cout << ans;
}




int main(){
    cin.tie(0); ios::sync_with_stdio(0);
    int T = 1, aa;
    // cin >> T;aa=T;
    while(T--){
        // cout << "Case #" << aa-T << ": ";
        solve();
        cout << '\n';
    }
    return 0;
 
}

Compilation message (stderr)

startrek.cpp: In function 'int main()':
startrek.cpp:95:16: warning: unused variable 'aa' [-Wunused-variable]
   95 |     int T = 1, aa;
      |                ^~

• Subtask 1: 0 / 0
• Subtask 2: 7 / 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