#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T, typename U> using ordered_map = tree<T, U, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<ll> vll;
#define FOR(i,j,k,in) for(int i=(j); i < (k);i+=in)
#define FORD(i,j,k,in) for(int i=(j); i >=(k);i-=in)
#define REP(i,b) FOR(i,0,b,1)
#define REPD(i,b) FORD(i,b,0,1)
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define all(x) begin(x), end(x)
#define MANY_TESTS int tcase; cin >> tcase; while(tcase--)
const double EPS = 1e-9;
const int MOD = 1e9+7;
const ll INFF = 1e18;
const int INF = 1e9;
const ld PI = acos((ld)-1);
const vi dy = {1, 0, -1, 0, -1, 1, 1, -1};
const vi dx = {0, 1, 0, -1, -1, 1, -1, 1};
void DBG(){cout << "]" << endl;}
template<typename T, typename ...U> void DBG(const T& head, const U... args){ cout << head << "; "; DBG(args...); }
#define dbg(...) cout << "Line(" << __LINE__ << ") -> [" << #__VA_ARGS__ << "]: [", DBG(__VA_ARGS__);
#define chk() cout << "Check at line(" << __LINE__ << ") hit." << endl;
template<class T, unsigned int U>
ostream& operator<<(ostream& out, const array<T, U> &v){out << "["; REP(i, U) out << v[i] << ", "; out << "]"; return out;}
template <class T, class U>
ostream& operator<<(ostream& out, const pair<T, U> &par) {out << "[" << par.first << ";" << par.second << "]"; return out;}
template <class T>
ostream& operator<<(ostream& out, const set<T> &cont) { out << "{"; for( const auto &x:cont) out << x << ", "; out << "}"; return out; }
template <class T, class U>
ostream& operator<<(ostream& out, const map<T, U> &cont) {out << "{"; for( const auto &x:cont) out << x << ", "; out << "}"; return out; }
template<class T>
ostream& operator<<(ostream& out, const vector<T> &v){ out << "["; REP(i, v.size()) out << v[i] << ", "; out << "]"; return out;}
template<class T>
istream& operator>>(istream& in, vector<T> &v){ for(auto &x : v) in >> x; return in; }
const int mod = (int)1e9+7;
struct Mint{
int val;
Mint(){ val = 0; }
Mint(long long v){
val = (-mod <= v && v < mod) ? v : v%mod;
if(val < 0) val+= mod;
}
// operators
Mint operator-(){return Mint(-val);}
Mint& operator+=(const Mint& other){ val+=other.val; if(val >= mod) val-=mod; return *this;}
Mint& operator-=(const Mint& other){ val-=other.val; if(val < 0) val+=mod; return *this;}
Mint& operator*=(const Mint& other){ val=((long long)val*other.val)%mod; return *this;}
friend Mint binpow(Mint a, long long p){
Mint res(1);
while(p > 0){
if(p&1) res*=a; a*=a; p>>=1;
}
return res;
}
friend Mint inv(Mint a){ return binpow(a, mod-2); }
Mint& operator/=(const Mint &other){ return (*this)*=inv(other);}
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; }
bool operator==(const Mint &other){return val == other.val;}
bool operator!=(const Mint &other){return val != other.val;}
bool operator<(const Mint &other){return val < other.val;}
// io
friend ostream& operator<<(ostream &out, Mint &a){ return out << a.val; }
};
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
ll n, d;
cin >> n >> d;
vector<vector<int>> g(n);
REP(i, n - 1){
int u, v;
cin >> u >> v;
u--; v--;
g[u].pb(v);
g[v].pb(u);
}
auto solve_subtask1 = [&](){
if(d % 2 == 0){
Mint ans = binpow(Mint(2), d);
cout << ans << "\n";
}
else{
cout << 0 << "\n";
}
};
auto solve = [&](){
vector<int> dp(n, 0);
function<void(int, int)> dfs = [&](int v, int p){
for(int x : g[v]){
if(x == p) continue;
dfs(x, v);
dp[v] |= (!dp[x] ? 1 : 0);
}
};
vector<int> new_dp = dp;
REP(i, n){
fill(all(dp), 0);
dfs(i, -1);
new_dp[i] = dp[i];
}
fill(all(dp), 0);
dfs(0, -1);
/*
function<void(int, int)> reroot = [&](int v, int p){
vector<int> pref(g[v].size(), 0);
vector<int> suf(g[v].size(), 0);
int cnt = 0;
for(int x : g[v]){
if(!dp[x])
pref[cnt] = 1;
if(cnt != 0)
pref[cnt] |= pref[cnt - 1];
cnt++;
}
reverse(all(g[v]));
cnt = 0;
for(int x : g[v]){
if(!dp[x])
suf[cnt] = 1;
if(cnt != 0)
suf[cnt] |= suf[cnt - 1];
cnt++;
}
int prevdp = dp[v];
new_dp[v] = pref[(int)g[v].size() - 1]; // ans;
reverse(all(suf));
reverse(all(g[v]));
REP(i, g[v].size()){
if(g[v][i] == p) continue;
dp[v] = pref[i - 1] | suf[i + 1];
reroot(g[v][i], v);
}
dp[v] = prevdp;
};
dfs(0, -1);
reroot(0, -1);
*/
int n_winning = 0;
int n_losing = 0;
REP(i, n){
if(new_dp[i] == 0)
n_losing++;
else
n_winning++;
}
vector<array<Mint, 2>> dp_placed(n);
function<void(int, int)> rec = [&](int v, int p){
int cnt_losing = 0;
for(int x : g[v]){
if(x == p) continue;
rec(x, v);
// how many winning?
if(dp[x]) cnt_losing++;
dp_placed[v][1] += dp_placed[x][0];
}
if(cnt_losing == 0){
for(int x : g[v]){
if(x == p) continue;
dp_placed[v][0] += dp_placed[x][1];
}
}
else if(cnt_losing > 1){
dp_placed[v][0] = Mint(0);
}
else{
for(int x : g[v]){
if(dp[x]){
dp_placed[v][0] = dp_placed[x][1];
}
}
}
// now what if I place edge
if(dp[v]){ // winning
dp_placed[v][1] += Mint(n);
}
else{
dp_placed[v][1] += Mint(n_losing);
dp_placed[v][0] += Mint(n_winning);
}
};
rec(0, -1);
cout << dp_placed[0][1] << "\n";
};
if(n == 2){
solve_subtask1();
}
else{
solve();
}
return 0;
}
Compilation message
startrek.cpp: In function 'Mint binpow(Mint, long long int)':
startrek.cpp:72:13: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
72 | if(p&1) res*=a; a*=a; p>>=1;
| ^~
startrek.cpp:72:29: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
72 | if(p&1) res*=a; a*=a; p>>=1;
| ^
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
364 KB |
Output is correct |
| 2 |
Incorrect |
20 ms |
364 KB |
Output isn't correct |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Incorrect |
1 ms |
364 KB |
Output isn't correct |
| 2 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
340 KB |
Output is correct |
| 2 |
Incorrect |
1 ms |
364 KB |
Output isn't correct |
| 3 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
340 KB |
Output is correct |
| 2 |
Incorrect |
1 ms |
364 KB |
Output isn't correct |
| 3 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
340 KB |
Output is correct |
| 2 |
Incorrect |
1 ms |
364 KB |
Output isn't correct |
| 3 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
340 KB |
Output is correct |
| 2 |
Incorrect |
1 ms |
364 KB |
Output isn't correct |
| 3 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
340 KB |
Output is correct |
| 2 |
Incorrect |
1 ms |
364 KB |
Output isn't correct |
| 3 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
364 KB |
Output is correct |
| 2 |
Incorrect |
20 ms |
364 KB |
Output isn't correct |
