| # | Time | Username | Problem | Language | Result | Execution time | Memory | 
|---|---|---|---|---|---|---|---|
| 1171442 | Zero_OP | Star Trek (CEOI20_startrek) | C++20 | 0 ms | 0 KiB | 
#include <bits/stdc++.h>
using namespace std;
#define FOR(i, l, r) for(int i = (l); i < (r); ++i)
#define ROF(i, r, l) for(int i = (r)-1; i >= (l); --i)
#define mp make_pair
#define mt make_tuple
#define ff first
#define ss second
#define all(v) begin(v), end(v)
#define rall(v) rbegin(v), rend(v)
#define sz(v) (int)v.size()
#define pb push_back
#define eb emplace_back
#define compact(v) v.erase(unique(all(v)), end(v))
#define dbg(x) "[" #x " = " << (x) << "]"
template<typename T>
      bool minimize(T& a, const T& b){
            if(a > b) return a = b, true;
            return false;
      }
template<typename T>
      bool maximize(T& a, const T& b){
            if(a < b) return a = b, true;
            return false;
      }
typedef long long ll;
using db = double;
using ull = unsigned long long;
using pi = pair<int, int>;
using pl = pair<ll, ll>;
using pd = pair<db, db>;
using vi = vector<int>;
using vl = vector<ll>;
using vc = vector<char>;
using vd = vector<db>;
using vb = vector<bool>;
using vpi = vector<pi>;
using vpl = vector<pl>;
void setIO(){
      ios_base::sync_with_stdio(0); cin.tie(0);
#ifdef LOCAL
      freopen("task.inp", "r", stdin);
      freopen("task.out", "w", stdout);
#endif
}
const int MAX = 1e5 + 5;
const int mod = 1e9 + 7;
struct mint{
      int v;
      mint() : v(0) {}
      mint(int v) : v(v) {}
      mint& operator += (const mint& o){
            v += o.v;
            if(v >= mod) v -= mod;
            return *this;
      }
      mint& operator -= (const mint& o){
            v -= o.v;
            if(v < 0) v += mod;
            return *this;
      }
      mint& operator *= (const mint& o){
            v = 1LL * v * o.v % mod;
            return *this;
      }
      mint power(ll n) const {
            mint res(1), base = *this;
            for(; n > 0; n >>= 1, base *= base) if(n & 1) res *= base;
            return res;
      }
      mint inv() const {
            return power(mod - 2);
      }
      mint& operator /= (const mint& o){
            return *this *= o.inv();
      }
      friend mint operator + (mint a, const mint& b){ return a += b; }
      friend mint operator - (mint a, const mint& b){ return a -= b; }
      friend mint operator * (mint a, const mint& b){ return a *= b; }
      friend mint operator / (mint a, const mint& b){ return a /= b; }
      friend ostream& operator << (ostream& out, const mint& o){ return out << o.v; }
};
int N;
ll D;
vi adj[MAX * 2];
namespace subtask2{
      bool dp[MAX * 2];
      void dfs(int u, int p){
            dp[u] = false;
            for(auto v : adj[u]) if(v != p){
                  dfs(v, u);
                  if(!dp[v]) dp[u] = 1;
            }
      }
      bool check(){
            dfs(0, -1);
            return dp[0];
      }
      void solve(){
            FOR(i, 0, N){
                  for(auto j : adj[i]) adj[i+N].pb(j+N);
            }
            int ans = 0;
            FOR(i, 0, N){
                  FOR(j, 0, N){
                        adj[i].pb(j+N);
//                        cout << dbg(i) << dbg(j+N) << '\n';
                        ans += check();
                        adj[i].pop_back();
                  }
            }
            cout << ans << '\n';
      }
}
namespace subtaskD1{
      bool dp[MAX], if_start_from[MAX], above[MAX];
      int num_zeroes[MAX], num_ones[MAX];
      void calc(int u, int p){
            dp[u] = false;
            for(auto v : adj[u]) if(v != p){
                  calc(v, u);
                  if(!dp[v]) dp[u] = true;
                  num_zeroes[u] += (dp[v] == 0);
                  num_ones[u] += (dp[v] == 1);
            }
      }
      void reroot(int u, int p){
            vi pref, suff;
            for(auto v : adj[u]) if(v != p){
                  pref.pb(dp[v]);
                  suff.pb(dp[v]);
            }
            for(int i = 0, j = sz(suff)-1; i < sz(pref); ++i, --j){
                  if(i > 0) pref[i] &= pref[i-1];
                  if(j+1<sz(suff)) suff[j] &= suff[j+1];
            }
            int i = 0;
            for(auto v : adj[u]) if(v != p){
                  int val = above[u];
                  if(i>0 && !pref[i-1]) val = true;
                  if(i+1<sz(suff) && !suff[i+1]) val = true;
//                  cout << dbg(v) << dbg(val) << dbg(above[u]) << '\n';
                  above[v] = !val;
                  if_start_from[v] = dp[v];
                  if(!dp[v] && !val) if_start_from[v] = true;
                  ++i;
                  reroot(v, u);
            }
      }
      int n_win_roots, n_lose_roots;
      void calc_ways(int u, int p){
            for(auto v : adj[u]) if(v != p){
                  //we can only change from dp[u] = false to dp[u] = true
                  
                  if(dp[u]){
                        
                  }
            }
      }
      void solve(){
            calc(0, -1);
            if_start_from[0] = dp[0];
            above[0] = false;
            reroot(0, -1);
            n_win_roots = 0, n_lose_roots = 0;
            FOR(i, 0, N){
                  if(!if_start_from[i]) ++n_lose_roots;
                  else ++n_win_roots;
//                  calc(i, -1);
//                  cout << dbg(i) << dbg(if_start_from[i]) << dbg(dp[i]) << '\n';
//                  assert(if_start_from[i] == dp[i]);
            }
            calc_ways(0, -1);
      }
}
int main(){
      setIO();
      cin >> N >> D;
      FOR(i, 0, N-1){
            int u, v;
            cin >> u >> v;
            --u, --v;
            adj[u].pb(v);
            adj[v].pb(u);
      }
      if(N == 2){
            cout << mint(4).power(D) << '\n';
            return 0;
      }
      if(D == 1){
            return subtaskD1::solve(), 0;
      }
      return 0;
}
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