func_code_string: #include <bits/stdc++.h>   using namespace std;   long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }   const int MOD = 998244353;   const int PRIMROOT = 3;   int pw(int x, int n) {     int ret = 1;     while (true) {       if (n & 1) ret = (long long)ret * x % MOD;       if ((n >>= 1) == 0) return ret;       x = (long long)x * x % MOD;     }   }   struct NTT {     void _init(int n, int &nlog, int &g, vector<int> &w) {       assert((MOD - 1) % n == 0);       g = pw(PRIMROOT, (MOD - 1) / n);       nlog = 0;       while ((1 << nlog) < n) ++nlog;       if ((1 << nlog) != n) nlog = -1;       w = vector<int>(n);       w[0] = 1;       for (int i = (1); i < (n); ++i) w[i] = (long long)w[i - 1] * g % MOD;     }     int fftn, fftnlog;     int fftg;     vector<int> fftw;     void initfft(int n) {       fftn = n, _init(fftn, fftnlog, fftg, fftw);       assert(fftnlog != -1);     }     void fft(int *a, int n, bool inv) {       if (n != fftn) initfft(n);       for (int i = 1, j = n >> 1, k, l = j; i < n; j ^= k, ++i) {         if (i < j) swap(a[i], a[j]);         for (k = l; k & j; j ^= k, k >>= 1)           ;       }       int *l, *r, *w;       if (inv) reverse(fftw.begin() + 1, fftw.end());       for (int hlen = 1, step = n >> 1; hlen < n; hlen <<= 1, step >>= 1) {         for (int i = 0; i < n; i += (hlen << 1)) {           l = a + i, r = l + hlen, w = fftw.data();           for (int off = (0); off < (hlen); ++off) {             int v = (long long)(*r) * (*w) % MOD;             *r = (*l) - v;             if (*r < 0) *r += MOD;             *l = (*l) + v;             if (*l >= MOD) *l -= MOD;             ++l, ++r, w += step;           }         }       }       if (inv) reverse(fftw.begin() + 1, fftw.end());       if (inv) {         int mlt = pw(n, MOD - 2);         for (int i = (0); i < (n); ++i) a[i] = (long long)a[i] * mlt % MOD;       }     }     void mult(const int *a, int na, const int *b, int nb, int *c, int nc) {       int sz = 1;       while (sz < na + nb - 1) sz *= 2;       vector<int> A(sz, 0), B(sz, 0);       for (int i = (0); i < (na); ++i) A[i] = a[i];       for (int i = (0); i < (nb); ++i) B[i] = b[i];       fft(A.data(), sz, false);       fft(B.data(), sz, false);       for (int i = (0); i < (sz); ++i) A[i] = (long long)A[i] * B[i] % MOD;       fft(A.data(), sz, true);       for (int i = (0); i < (nc); ++i) c[i] = i < sz ? A[i] : 0;     }   };   NTT ntt;   void inc(int &a, int b) {     if ((a += b) >= MOD) a -= MOD;   }   vector<int> add(const vector<int> &a, const vector<int> &b) {     vector<int> c(max(((int)(a).size()), ((int)(b).size())), 0);     for (int i = (0); i < (((int)(c).size())); ++i) {       if (i < ((int)(a).size())) inc(c[i], a[i]);       if (i < ((int)(b).size())) inc(c[i], b[i]);     }     return c;   }   vector<int> mult(const vector<int> &a, const vector<int> &b) {     vector<int> c(((int)(a).size()) + ((int)(b).size()) - 1, 0);     ntt.mult(a.data(), ((int)(a).size()), b.data(), ((int)(b).size()), c.data(),              ((int)(c).size()));     return c;   }   const int MAXN = 100000;   int n;   long long want;   vector<int> adj[MAXN];   int par[MAXN];   int sz[MAXN];   int dep[MAXN];   int heavy[MAXN];   vector<int> ways[MAXN];   vector<int> waysnonheavy[MAXN];   void dfsinit(int at) {     dep[at] = par[at] == -1 ? 0 : dep[par[at]] + 1, sz[at] = 1, heavy[at] = -1;     for (int i = (0); i < (((int)(adj[at]).size())); ++i) {       int to = adj[at][i];       if (to == par[at]) continue;       par[to] = at;       dfsinit(to);       sz[at] += sz[to];       if (heavy[at] == -1 || sz[to] > sz[heavy[at]]) heavy[at] = to;     }   }   int path[MAXN], npath;   pair<vector<int>, vector<int>> rec(int l, int r) {     if (l == r) {       int at = path[l];       return make_pair(waysnonheavy[at], waysnonheavy[at]);     } else {       int m = l + (r - l) / 2;       auto res1 = rec(l, m);       auto res2 = rec(m + 1, r);       return make_pair(add(res1.first, mult(res1.second, res2.first)),                        mult(res1.second, res2.second));     }   }   int lst[MAXN], nlst;   vector<int> tmp[2 * MAXN];   int ntmp;   priority_queue<pair<int, int>> pq;   void calcways(int at);   void calcwaysnonheavy(int at) {     for (int i = (0); i < (((int)(adj[at]).size())); ++i) {       int to = adj[at][i];       if (to == par[at] || to == heavy[at]) continue;       calcways(to);     }     nlst = 0;     for (int i = (0); i < (((int)(adj[at]).size())); ++i) {       int to = adj[at][i];       if (to == par[at] || to == heavy[at]) continue;       lst[nlst++] = to;     }     ntmp = nlst;     pq = priority_queue<pair<int, int>>();     for (int i = (0); i < (nlst); ++i)       tmp[i] = ways[lst[i]], pq.push(make_pair(-((int)(tmp[i]).size()), i));     while (((int)(pq).size()) >= 2) {       int a = pq.top().second;       pq.pop();       int b = pq.top().second;       pq.pop();       int c = ntmp++;       tmp[ntmp] = mult(tmp[a], tmp[b]),       pq.push(make_pair(-((int)(tmp[c]).size()), c));     }     waysnonheavy[at] =         ((int)(pq).size()) == 1 ? tmp[pq.top().second] : vector<int>(1, 1);     waysnonheavy[at].insert(waysnonheavy[at].begin(), 0);     for (int i = (0); i < (ntmp); ++i) tmp[i].clear();   }   void calcways(int at) {     for (int x = at; x != -1; x = heavy[x]) calcwaysnonheavy(x);     npath = 0;     for (int x = at; x != -1; x = heavy[x]) path[npath++] = x;     ways[at] = rec(0, npath - 1).first;     inc(ways[at][0], 1);   }   int inv[MAXN + 1];   void run() {     scanf( %d%lld , &n, &want);     for (int i = (0); i < (n - 1); ++i) {       int a, b;       scanf( %d%d , &a, &b);       --a, --b;       adj[a].push_back(b);       adj[b].push_back(a);     }     par[0] = -1;     dfsinit(0);     calcways(0);     int ret = 0, mlt = 1;     inv[1] = 1;     for (int i = (2); i < (((int)(ways[0]).size())); ++i)       inv[i] = (long long)(MOD - MOD / i) * inv[MOD % i] % MOD;     for (int i = (1); i < (((int)(ways[0]).size())); ++i) {       ret = (ret + (long long)ways[0][i] * mlt) % MOD;       mlt = (long long)mlt * (want % MOD + i) % MOD * inv[i] % MOD;     }     printf( %d n , ret);   }   int main() {     run();     return 0;   }   