#include "bits/stdc++.h"
using namespace std;
vector<int> adj[100005];
int dp[100005], up[100005], cnt[100005], n, sub[100005], rev[100005], tot[100005], tmp[100005];
void dfs(int u, int p = -1){
for(auto v : adj[u]){
if(v == p) continue;
dfs(v, u);
if(dp[v] == 0){
dp[u] = 1;
tmp[u] += sub[v];
cnt[u]++;
}
tot[u] += sub[v];
}
if(cnt[u] > 1) sub[u] = 0;
if(cnt[u] == 1) sub[u] = tmp[u];
if(cnt[u] == 0) sub[u] = tot[u] + 1;
}
void re(int u, int p = -1){
tot[u] += rev[u];
if(up[u] == 0){
tmp[u] += rev[u];
dp[u] = 1, cnt[u]++;
}
if(cnt[u] > 1) sub[u] = 0;
if(cnt[u] == 1) sub[u] = tmp[u];
if(cnt[u] == 0) sub[u] = tot[u] + 1;
for(auto v : adj[u]){
if(v == p) continue;
tot[u] -= sub[v];
if(!dp[v]){
cnt[u]--; tmp[u] -= sub[x];
}
if(cnt[u]) up[v] = 1;
if(cnt[u] > 1) rev[v] = 0;
if(cnt[u] == 1) rev[v] = tmp[u];
if(cnt[u] == 0) rev[v] = tot[u] + 1;
tot[u] += sub[x];
if(!dp[x]){
cnt[u]++; tmp[u] += sub[x];
}
re(x, u);
}
}
int bp(int a, int b){
int c = 1;
while(b){
if(b & 1) c = c * 1ll * a % mod;
a = a * 1ll * a % mod, b >>= 1;
}
return c;
}
signed main(){
ios::sync_with_stdio(0); cin.tie(0);
int d; cin>>n>>d;
for(int i=1;i<n;i++){
int a, b; cin>>a>>b;
adj[a].push_back(b);
adj[b].push_back(a);
}
dfs(1);
up[1] = 1;
re(1);
int E = 0, L = 0;
for(int i=1;i<=n;i++){
if(dp[i] == 1) E = (E + sub[i]) % mod;
else E = (E - sub[i] + mod) % mod, L++;
}
int F = n * 1ll * n % mod;
int res = (bp(F, d) - bp(E, d) + mod) % mod * 1ll * bp((F - E + mod) % mod, mod - 2) % mod;
res = res * 1ll * L % mod;
if(dp[1]) res = res * 1ll * sub[1] % mod;
else res = (bp(F, d) - res * 1ll * sub[1] % mod + mod) % mod;
res = (bp(F, d) - res + mod) % mod;
res = (res % mod + mod) % mod;
cout<<res<<"\n";
}
Compilation message
startrek.cpp: In function 'void re(int, int)':
startrek.cpp:36:28: error: 'x' was not declared in this scope
36 | cnt[u]--; tmp[u] -= sub[x];
| ^
startrek.cpp:42:17: error: 'x' was not declared in this scope
42 | tot[u] += sub[x];
| ^
startrek.cpp: In function 'int bp(int, int)':
startrek.cpp:53:31: error: 'mod' was not declared in this scope; did you mean 'modf'?
53 | if(b & 1) c = c * 1ll * a % mod;
| ^~~
| modf
startrek.cpp:54:21: error: 'mod' was not declared in this scope; did you mean 'modf'?
54 | a = a * 1ll * a % mod, b >>= 1;
| ^~~
| modf
startrek.cpp: In function 'int main()':
startrek.cpp:73:37: error: 'mod' was not declared in this scope; did you mean 'modf'?
73 | if(dp[i] == 1) E = (E + sub[i]) % mod;
| ^~~
| modf
startrek.cpp:74:26: error: 'mod' was not declared in this scope; did you mean 'modf'?
74 | else E = (E - sub[i] + mod) % mod, L++;
| ^~~
| modf
startrek.cpp:77:24: error: 'mod' was not declared in this scope; did you mean 'modf'?
77 | int F = n * 1ll * n % mod;
| ^~~
| modf
