标签:括号 怎么 dig printf 递推 情况 cin mes etc
\(NTT\) 神仙题,强烈建议先自己推一推式子再看题解,顺便orz memset0。
题面没看太懂,大概意思就是求有n个不同的小球,放进 \(1\)、\(2\)、$……n $ 个不同的盒子,不可空的情况下,期望用了几个盒子。
按照套路,我们应该分别求出总的方案数\(g_i\)和总共用的盒子数 \(f_i\) ,答案就是 \(f_i\times g_i^{-1}\)。
我们先来求总的方案数,设 \(S\) 是第二类斯特林数,则
\(\displaystyle g_n=\sum_{i=1}^nS[n][i]\times i!\)
就是我们枚举用了几个盒子,在乘上 \(i!\) 将盒子看成是不一样的。
但是我们没办法进行下去了,我们考虑一下递推式。
\(\displaystyle g_n=\sum_{i=1}^nC_n^ig_{n-i}\)
含义就是我们枚举第一个盒子里都有哪些球,再乘上剩下的球的方案数 \(g_{n-i}\) 。我们尝试化简一下式子。
\(\displaystyle g_n=\sum_{i=1}^n \frac{n!}{i!(n-i)!}g_{n-i}\)
\(\displaystyle \frac{g_n}{n!}=\sum_{i=1}^n \frac{1}{i!}\frac{g_{n-i}}{(n-i)!}\)
设 \(\displaystyle G_n=\frac{g_n}{n!}\) \(\displaystyle H_n=\frac{1}{n!}\) ,注意 \(H_0=1,G_0=1\) 那么
\(\displaystyle G_n=\sum_{i=1}^nH_iG_{n-i}\)
有点像分治 \(FFT\) ,但我们会多项式求逆,我们按照分治 \(FFT\) 的套路来。
设 \(\displaystyle G=\sum_{i=0}^{\infty}G_ix^i\) \(\displaystyle H=\sum_{i=0}^{\infty}H_ix^i\)
\(\displaystyle G*H=\sum_{i=0}^{\infty}(\sum_{j+k=i}G_jH_k)x^i\)
当 \(i>0\) 时,\(\displaystyle \sum_{j+k=i}G_jH_k=\sum_{j=0}^iH_jG_{i-j}=\sum_{j=1}^iH_jG_{i-j}+H_0G_i=2G_i\)
当 \(i=0\) 时,\(G_0H_0=1=2G_0-1\)
所以
\(G*H=2G-1\)
\(G=(2-H)^{-1}\)
多项式求逆一波带走。
我们再来看看怎么求总共的盒子数。
\(\displaystyle f_n=g_n+\sum_{i=1}^nC_n^if_{n-i}\)
含义就是首先我们每一种方案至少有一个盒子,所以我们先加上 \(g_n\) ,对于剩下的 \(n-i\) 个小球,对答案的贡献就是 \(f_{n-i}\) ,因为第一个盒子有 \(C_n^i\) 种选法,所以就是 \(\displaystyle f_n=g_n+\sum_{i=1}^nC_n^if_{n-i}\)
我们再来搞一搞。现将 \(\displaystyle g_n=\sum_{i=1}^nC_n^ig_{n-i}\) 带进去。
\(\displaystyle f_n=\sum_{i=1}^nC_n^ig_{n-i}+\sum_{i=1}^nC_n^if_{n-i}\)
\(\displaystyle \frac{f_n}{n!}=\sum_{i=1}^{n}\frac{1}{i!}\frac{g_{n-i}}{(n-i)!}+\sum_{i=1}^{n}\frac{1}{i!}\frac{f_{n-i}}{(n-i)!}\)
设 \(\displaystyle F_n=\frac{f_n}{n!}\)
\(\displaystyle F_n=\sum_{i=1}^nH_i(F_{n-i}+G_{n-i})\)
设 \(\displaystyle F=\sum_{i=0}^{\infty}F_ix^i\)
那么
\(\displaystyle H*(F+G)=\sum_{i=0}^{\infty}(\sum_{j+k=i}((F_k+G_k)H_j))x^i\) (注意看清大括号)
当 \(i>0\) 时,\(\displaystyle \sum_{j+k=i}(F_k+G_k)H_j=\sum_{j=0}^i(F_{i-j}+G_{i-j})H_{j}=\sum_{j=1}^i(F_{i-j}+G_{i-j})H_{j}+F_i+G_i=2F_i+G_i\)
当 \(i=0\) 时,\((F_0+G_0)H_0=1=2F_0+G_0\)
所以
\(\displaystyle H*(F+G)=2F+G\)
\(\displaystyle F=\frac{G-GH}{H-2}\)
因为 \(G=(2-H)^{-1}\)
所以 \(G-GH=1-G\)
带进去
\(\displaystyle F=(1-G)(H-2)^{-1}\)
最后答案就是 \(F_nG_n^{-1}\)
推这些式子我竟然推了一下午,我还是太菜了。
#include<iostream>
#include<cstdio>
#define DB double
#define LL long long
using namespace std;
int T, n;
const int N = 400010, M = 100000, mod = 998244353, YY = 3, YYinv = (mod + 1) / 3;
int r[N];
LL jc[N], inv[N], H[N], g[N], G[N], F[N];
inline int read()
{
int res = 0; char ch = getchar(); bool XX = false;
for (; !isdigit(ch); ch = getchar())(ch == '-') && (XX = true);
for (; isdigit(ch); ch = getchar())res = (res << 3) + (res << 1) + (ch ^ 48);
return XX ? -res : res;
}
void Write(int x, int opt)
{
if (opt && !x)putchar('0');
if (!x)return;
Write(x / 10, 0);
putchar((x - x / 10 * 10) + '0');
}
LL ksm(LL a, LL b, LL mod)
{
LL res = 1;
for (; b; b >>= 1, a = a * a % mod)
if (b & 1)res = res * a % mod;
return res;
}
void NTT(LL *A, int lim, int opt)
{
for (int i = 0; i < lim; ++i)
r[i] = (r[i >> 1] >> 1) | ((i & 1) ? (lim >> 1) : 0);
for (int i = 0; i < lim; ++i)
if (i < r[i])swap(A[i], A[r[i]]);
int len;
LL wn, w, x, y;
for (int mid = 1; mid < lim; mid <<= 1)
{
len = mid << 1;
wn = ksm(opt == 1 ? YY : YYinv, (mod - 1) / len, mod);
for (int j = 0; j < lim; j += len)
{
w = 1;
for (int k = j; k < j + mid; ++k, w = w * wn % mod)
{
x = A[k]; y = A[k + mid] * w % mod;
A[k] = (x + y) % mod;
A[k + mid] = (x - y + mod) % mod;
}
}
}
if (opt == 1)return;
int ni = ksm(lim, mod - 2, mod);
for (int i = 0; i < lim; ++i)A[i] = A[i] * ni % mod;
}
LL c[N];
void INV(int siz, LL *A, LL *B)
{
if (siz == 1)
{
B[0] = ksm(A[0], mod - 2, mod);
return;
}
INV((siz + 1) >> 1, A, B);
int lim = 1;
while (lim < (siz << 1))lim <<= 1;
for (int i = 0; i < siz; ++i)c[i] = A[i];
for (int i = siz; i < lim; ++i)c[i] = 0;
NTT(c, lim, 1); NTT(B, lim, 1);
for (int i = 0; i < lim; ++i)B[i] = B[i] * (2 - c[i] * B[i] % mod + mod) % mod;
NTT(B, lim, -1);
for (int i = siz; i < lim; ++i)B[i] = 0;
}
void MUL(LL *A, int n, LL *B, int m)
{
int lim = 1;
while (lim < (n + m))lim <<= 1;
NTT(A, lim, 1); NTT(B, lim, 1);
for (int i = 0; i < lim; ++i)A[i] = A[i] * B[i] % mod;
NTT(A, lim, -1);
}
void YYCH()
{
inv[0] = inv[1] = jc[0] = jc[1] = 1;
for (int i = 2; i <= M; ++i)jc[i] = jc[i - 1] * i % mod;
inv[M] = ksm(jc[M], mod - 2, mod);
for (int i = M - 1; i >= 1; --i)inv[i] = inv[i + 1] * (i + 1) % mod;
for (int i = 0; i <= M; ++i)
{
H[i] = inv[i];
g[i] = mod - H[i];
}
g[0] += 2;
INV(M + 1, g, G);
for (int i = 0; i <= M; ++i)
{
F[i] = G[i]; g[i] = G[i];
}
F[0]--;
MUL(F, M, g, M);
}
int main()
{
YYCH();
cin >> T;
while (T--)
{
n = read();
printf("%lld\n", F[n]*ksm(G[n], mod - 2, mod) % mod);
}
return 0;
}
标签:括号 怎么 dig printf 递推 情况 cin mes etc
原文地址:https://www.cnblogs.com/wljss/p/12036854.html