Language: C++

#define DEBUGGING
#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define ALL(V) (V).begin(), (V).end()
#define ALLR(V) (V).rbegin(), (V).rend()

template <typename T> using V = vector<T>;
template <typename T> using VV = V<V<T>>;
using ll = int64_t;
using ull = uint64_t;
using PLL = pair<ll, ll>;

template <typename T> const T& var_min(const T &t) { return t; }
template <typename T> const T& var_max(const T &t) { return t; }
template <typename T, typename... Tail> const T& var_min(const T &t, const Tail&... tail) { return min(t, var_min(tail...)); }
template <typename T, typename... Tail> const T& var_max(const T &t, const Tail&... tail) { return max(t, var_max(tail...)); }
template <typename T, typename... Tail> void chmin(T &t, const Tail&... tail) { t = var_min(t, tail...); }
template <typename T, typename... Tail> void chmax(T &t, const Tail&... tail) { t = var_max(t, tail...); }
template <typename T> const T& clamp(const T &t, const T &low, const T &high) { return max(low, min(high, t)); }
template <typename T> void chclamp(T &t, const T &low, const T &high) { t = clamp(t, low, high); }

namespace init__ {

struct InitIO {
    InitIO() {
        cin.tie(nullptr);
        ios_base::sync_with_stdio(false);
        cout << fixed << setprecision(30);
    }
} init_io;

}

#ifdef DEBUGGING
#include "../debug.cpp"
#else
#define DEBUG(...) 0
#define DEBUG_SEPARATOR_LINE 0
#endif

template <typename T>
T make_v(T init) { return init; }

template <typename T, typename... Tail>
auto make_v(T init, size_t s, Tail... tail) {
#define rec make_v(init, tail...)
    return V<decltype(rec)>(s, rec);
#undef rec
}

template <ll MOD>
class Combination {
private:
    template <typename T> using V = vector<ll>;
    ll N;
    V<ll> factv, rfactv;

public:
    /*
     * MOD must be a prime number.
     */
    Combination<MOD> (ll N)
        : N(N), 
          factv(N + 1, 1),
          rfactv(N + 1)
    {
        for(ll i = 1; i <= N; i++) {
            factv[i] = factv[i - 1] * i % MOD;
        }
        for(ll i = 0; i <= N; i++) {
            rfactv[i] = pow(factv[i], MOD - 2);
        }
    }

    ll fact(ll n) {
        return factv[n];
    }

    ll rfact(ll n) {
        return rfactv[n];
    }

    ll pow(ll a, ll b) {
        return b ? (b & 1 ? a : 1) * pow(a * a % MOD, b / 2) % MOD : 1;
    }

    ll comb(ll n, ll k) {
        return factv[n] * rfactv[n - k] % MOD * rfactv[k] % MOD;
    }

    ll perm(ll n, ll k) { return factv[n] * rfactv[n - k] % MOD; }
};

const ll mod = 1e9 + 7;

VV<ll> edges;
ll N, K;

Combination<mod> comb(1e6 + 10);

ll dfs(ll now, ll pre) {
    ll csum = edges[now].size() - 1;
    ll ret;
    if(pre == -1) {
        csum += 2;
        if(K < csum) return 0;
        ret = comb.perm(K, csum);
    } else {
        if(K - 2 < csum) return 0;
        ret = comb.perm(K - 2, csum);
    }
    for(ll nxt : edges[now]) {
        if(pre == nxt) continue;
        (ret *= dfs(nxt, now)) %= mod;
    }
    return ret;
}

int main() {
    cin >> N >> K;
    edges.resize(N);
    for(ll i = 1; i < N; i++) {
        ll a, b;
        cin >> a >> b;
        a--; b--;
        edges[a].push_back(b);
        edges[b].push_back(a);
    }

    cout << dfs(0, -1) << endl;
    return 0;
}


Compiler output:
a.cc:36:10: fatal error: ../debug.cpp: No such file or directory
   36 | #include "../debug.cpp"
      |          ^~~~~~~~~~~~~~
compilation terminated.