/*
Code written by Talant I.D.
*/
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair <int, int> pii;
typedef pair <ll, ll> pll;
#define precision(n) fixed << setprecision(n)
#define pb push_back
#define ub upper_bound
#define lb lower_bound
#define mp make_pair
#define eps (double)1e-9
#define PI 2*acos(0.0)
#define endl "\n"
#define sz(v) int((v).size())
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define do_not_disturb ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define OK cout << "OK" << endl;
const int mod = 1e9+7;
ll mode(ll a) {
a %= mod;
if (a < 0) a += mod;
return a;
}
ll subt(ll a, ll b) {
return mode(mode(a)-mode(b));
}
ll add(ll a, ll b) {
return mode(mode(a)+mode(b));
}
ll mult(ll a, ll b) {
return mode(mode(a)*mode(b));
}
ll binpow(ll a, ll b) {
ll res = 1;
while (b) {
if (b&1) res = mult(res, a);
a = mult(a, a);
b >>= 1;
}
return res;
}
const int N = 1e5+7;
int state[N], l, w, state_as_root[N];
int losing_children[N], critical[N], depth[N], par[N];
ll dp[N];
vector <int> graph[N];
bool dfs(int v, int p) {
par[v] = p;
depth[v] = depth[p]+1;
int cnt = 0, children = 0;
for (auto to : graph[v]) {
if (to == p) continue;
children++;
dfs(to, v);
cnt += (state[to] ? 1 : 0);
if (state[to]) losing_children[v]++;
}
if (children == 0) state[v] = 1;
else if (cnt) state[v] = 0;
else state[v] = 1;
return state[v];
}
void dfs2(int v, int p, int origin, int cnt = 0) {
if (cnt == depth[v] && state[v]) critical[origin]++;
if (state[v]) {
for (auto to : graph[v]) {
if (to == p) continue;
dfs2(to, v, origin, cnt+1);
}
}
else {
for (auto to : graph[v]) {
if (to == p) continue;
if (state[to]) {
dfs2(to, v, origin, (losing_children[v] == 1 ? cnt+1 : 0));
}
}
}
}
void dfs3(int v, int p) {
state_as_root[v] = state[v];
if (state[v]) l++;
else w++;
for (auto to : graph[v]) {
if (to == p) continue;
if (state[v] == false && state[to] == true) {
if (losing_children[v] == 1 && (state_as_root[par[v]] == false)) {
losing_children[v]--;
losing_children[to]++;
state[v] = 1;
state[to] = 0;
dfs3(to, v);
losing_children[v]++;
losing_children[to]--;
state[v] = 0;
state[to] = 1;
}
else {
dfs3(to, v);
}
}
else {
dfs3(to, v);
}
}
}
int main() {
do_not_disturb
int n;
ll d;
cin >> n >> d;
for (int i = 0; i < n-1; i++) {
int x, y;
cin >> x >> y;
graph[x].pb(y);
graph[y].pb(x);
}
if (n == 2) {
cout << binpow(2, d*2);
}
else if (d <= 1e5 && n <= 1000) {
depth[1] = -1;
dfs(1, 1);
dfs3(1, 1);
for (int i = 1; i <= n; i++) {
depth[i] = -1;
for (int j = 1; j <= n; j++) losing_children[j] = 0;
dfs(i, i);
dfs2(i, i, i);
}
dp[0] = l;
ll E = 0;
for (int i = 1; i <= n; i++) {
if (state[i]) {
E = subt(E, critical[i]);
}
else {
E = add(E, critical[i]);
}
}
for (int i = d-1; i >= 0; i--) {
dp[d-i] = add(mult(l, binpow(n, 2*(d-i))), mult(E, dp[d-i-1]));
}
cout << (state[1] ? mult(dp[d-1], critical[1]) : subt(binpow(n, 2*d), mult(dp[d-1], critical[1]))) << endl;
}
else if (d == 1) {
depth[1] = -1;
dfs(1, 1);
dfs2(1, 1, 1);
dfs3(1, 1);
if (state_as_root[1]) cout << mult(critical[1], l);
else cout << add(mult(n, w), mult(subt(n, critical[1]), l));
}
return 0;
}
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 2 |
Incorrect |
26 ms |
2752 KB |
Output isn't correct |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 2 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 3 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 4 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 5 |
Correct |
2 ms |
2636 KB |
Output is correct |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 2 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 3 |
Incorrect |
2 ms |
2636 KB |
Output isn't correct |
| 4 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 2 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 3 |
Incorrect |
2 ms |
2636 KB |
Output isn't correct |
| 4 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 2 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 3 |
Incorrect |
2 ms |
2636 KB |
Output isn't correct |
| 4 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 2 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 3 |
Incorrect |
2 ms |
2636 KB |
Output isn't correct |
| 4 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 2 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 3 |
Incorrect |
2 ms |
2636 KB |
Output isn't correct |
| 4 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
2 ms |
2636 KB |
Output is correct |
| 2 |
Incorrect |
26 ms |
2752 KB |
Output isn't correct |
