代码

#include<bits/stdc++.h>
#define maxn 4100
#define mod 998244353
#define rep(i,j,k) for(int i=(j),LIM=(k);i<=LIM;i++)
#define per(i,j,k) for(int i=(j),LIM=(k);i>=LIM;i--) 	
#define LL long long
using namespace std;

LL m;
int n;
int x[maxn],c[maxn],f[maxn][maxn],inv[maxn]={1,1},invf[maxn]={1,1},fac[maxn]={1,1},sz[maxn],lg[maxn];
int Wl,Wl2;
vector<int>G[maxn];

int C(int n,int m){ return fac[n] * 1ll * invf[m] % mod * invf[n-m] % mod; }
int Cb(int n,int m){
	int r = 1;
	for(int i=1;i<=m;i++)
		r = 1ll * r * (n-i+1) % mod;
	return 1ll * r * invf[m] % mod;
}

int w[maxn]={1},r[maxn];
int Pow(int b,int k){ int r=1;for(;k;k>>=1,b=1ll*b*b%mod) if(k&1) r=1ll*r*b%mod; return r; }

void init(int n){
	for(Wl=1;n>=Wl<<1;Wl<<=1);int pw=Pow(3,(mod-1)/(Wl2=Wl<<1));
	rep(i,1,Wl2) w[i]=1ll*w[i-1]*pw%mod,(i>1)&&(lg[i]=lg[i>>1]+1,inv[i]=1ll*(mod-mod/i)*inv[mod%i]%mod,
		fac[i]=1ll*fac[i-1]*i%mod,invf[i]=1ll*invf[i-1]*inv[i]%mod);
}

void upd(int &u){ u+=u>>31&mod; }
void NTT(int *A,int n,int tp){
	rep(i,1,n-1) i<(r[i]=(r[i>>1]>>1)|(i&1)<<lg[n]-1) && (swap(A[i],A[r[i]]),0);
	for(int L=1,B=Wl;L<n;L<<=1,B>>=1) for(int s=0;s<n;s+=L<<1) for(int k=s,x=0,t;k<s+L;k++,x+=B)
		t=1ll*w[tp==1?x:Wl2-x]*A[k+L]%mod,upd(A[k+L]=A[k]-t),upd(A[k]+=t-mod);
	if(tp^1) rep(i,0,n-1) A[i]=1ll*A[i]*inv[n]%mod;
}

void MUL(int A[],int B[],int C[],int n,int m){
	
	if(n*m<=4000){
		rep(i,0,n+m) C[i] = 0;
		rep(i,0,n){
			int j=0;
			for(;j+7<=m;j+=8)
				C[i+j] = (C[i+j] + 1ll * A[i] * B[j]) % mod,
				C[i+j+1] = (C[i+j+1] + 1ll * A[i] * B[j+1]) % mod,
				C[i+j+2] = (C[i+j+2] + 1ll * A[i] * B[j+2]) % mod,
				C[i+j+3] = (C[i+j+3] + 1ll * A[i] * B[j+3]) % mod,
				C[i+j+4] = (C[i+j+4] + 1ll * A[i] * B[j+4]) % mod,
				C[i+j+5] = (C[i+j+5] + 1ll * A[i] * B[j+5]) % mod,
				C[i+j+6] = (C[i+j+6] + 1ll * A[i] * B[j+6]) % mod,
				C[i+j+7] = (C[i+j+7] + 1ll * A[i] * B[j+7]) % mod;
			for(;j<=m;j++)
				C[i+j] = (C[i+j] + 1ll * A[i] * B[j]) % mod;
		} 
	}
	
	int L = 1<<lg[n+m]+1;
	static int sta[2][maxn];
	rep(i,0,L-1) sta[0][i]=i<=n?A[i]:0,sta[1][i]=i<=m?B[i]:0;
	NTT(sta[0],L,1),NTT(sta[1],L,1);
	rep(i,0,L-1) C[i] = 1ll * sta[0][i] * sta[1][i] % mod;
	NTT(C,L,-1);
}

void dfs(int u){
	sz[u] = 0;
	f[u][0] = 1;
	rep(i,0,G[u].size()-1){
		int v;dfs(v=G[u][i]);
		static int g[maxn]={};
		memset(g,0,4*(sz[u]+sz[v]+1));
		/*if(max(sz[u],sz[v]) <= 100){
			rep(j,0,sz[u]) if(f[u][j]) rep(k,0,sz[v]) if(f[v][k])
				rep(p,max(k,j),k+j) 
					g[p] = (g[p] + f[u][j] * 1ll * f[v][k] % mod * C(p,j) % mod * C(j,j+k-p)) % mod;
		}
		else{*/
			static int sta[2][maxn]={};
			rep(k,0,sz[v]){
				rep(p,0,k) sta[0][p] = 1ll * invf[p] * invf[k-p] % mod * f[v][k] % mod;
				MUL(sta[0],f[u],sta[1],k,sz[u]);
				rep(p,k,sz[u]+k)	g[p] = (g[p] + sta[1][p] * 1ll * fac[p] % mod * invf[p-k]) % mod;
			}
	//	}
		sz[u] += sz[v];
		rep(j,0,sz[u]) f[u][j] = g[j];
	}
	sz[u]++;
	per(i,sz[u],1) f[u][i] = (f[u][i-1] + f[u][i]) % mod;
	f[u][0] = 0;
}

int main(){
	scanf("%d%lld",&n,&m);m%=mod;
	init(n);
	rep(i,1,n) scanf("%d%d",&x[i],&c[i]),G[x[i]].push_back(i);
	int ans = 1;
	rep(i,0,G[0].size()-1){
		int v = G[0][i];
		dfs(v);
		int sm = 0;
		rep(j,1,sz[v]) sm = (1ll * Cb(m-c[v]+1,j) * f[v][j] + sm) % mod;
		ans = 1ll * ans * sm % mod;
	}
	printf("%d\n",(ans+mod)%mod);
}

评测结果