题目链接 2017 CCPC Hangzhou Problem G
题意描述很清晰。
考虑每个家庭有且仅有$k$对近亲的方案数：
$C(a, k) * C(b, k) * k!$
那么如果在第$1$个家庭里面选出$k_{1}$对近亲，在第$2$个家庭里面选出$k_{2}$对近亲......在第$n$个家庭里面选出$k_{n}$对近亲，
剩下那些人自由组合的话，那么最后这种方案至少会有$∑k$对近亲。
说是至少，因为同一个家庭里面没被强行选择的男女还是可能被组到了一起。
那么考虑如何求至少有$k$对近亲的方案数。
构造$n$个多项式，对于每个家庭，这个多项式为
$c_{0} + c_{1}x + c_{2}x^{2} + c_{3}x^{3} + c_{4}x^{4} + ... + c_{p}x^{p}$， $p = min(a, b)$
其中$c_{i}$这个系数为在这个家庭里面选出$i$对近亲的方案数。
那么只要把这$n$个多项式乘起来，得到的结果里面$x^{k}$的系数就是至少有$k$对近亲的方案数。
把$n$个多项式求出来用分治NTT即可，我用了启发式合并。
因为是至少，所以还要考虑容斥。
最后的答案就是$(-1)^{k}a_{k} * (m - k)!$，$m$为总人数
时间复杂度$O(nlog^{2}n)$
#include <bits/stdc++.h> using namespace std; #define rep(i, a, b) for (int i(a); i <= (b); ++i)
#define dec(i, a, b) for (int i(a); i >= (b); --i)
#define MP make_pair
#define fi first
#define se second typedef long long LL; const int N = 1e5 + 10;
const LL mod = 998244353;
const LL g = 3; vector <LL> v[N << 1];
LL x1[N << 1], x2[N << 1];
LL fac[N];
LL ans, flag;
int T;
int n, all, cnt;
int sz; inline LL Pow(LL a, LL b, LL mod){
LL ret(1);
for (; b; b >>= 1, (a *= a) %= mod) if (b & 1) (ret *= a) %= mod;
return ret;
} inline LL C(LL n, LL m){ return m > n ? 0 : fac[n] * Pow(fac[m] * fac[n - m] % mod, mod - 2, mod) % mod; } struct cmp{
bool operator ()(int a, int b){
return v[a].size() > v[b].size();
}
}; priority_queue <LL, vector <LL>, cmp> q; void change(LL y[], int len){
for (int i = 1, j = len / 2; i < len - 1; i++){
if (i < j) swap(y[i], y[j]);
int k = len / 2;
while (j >= k){
j -= k;
k /= 2;
}
if (j < k) j += k;
}
} void ntt(LL y[], int len, int on){
change(y, len);
for (int h = 2; h <= len; h <<= 1){
LL wn = Pow(g, (mod - 1) / h, mod);
if (on == -1) wn = Pow(wn, mod - 2, mod);
for (int j = 0; j < len; j += h){
LL w = 1ll;
for (int k = j; k < j + h / 2; k++){
LL u = y[k];
LL t = w * y[k + h / 2] % mod;
y[k] = (u + t) % mod;
y[k + h / 2] = (u - t + mod) % mod;
w = w * wn % mod;
}
}
} if (on == -1){
LL t = Pow(len, mod - 2, mod);
rep(i, 0, len - 1) y[i] = y[i] * t % mod;
}
} void mul(vector <LL> &a, vector <LL> &b, vector <LL> &c){
int len = 1;
int sz1 = a.size(), sz2 = b.size(); while (len <= sz1 + sz2 - 1) len <<= 1; rep(i, 0, sz1 - 1) x1[i] = a[i];
rep(i, sz1, len) x1[i] = 0; rep(i, 0, sz2 - 1) x2[i] = b[i];
rep(i, sz2, len) x2[i] = 0; ntt(x1, len, 1);
ntt(x2, len, 1); rep(i, 0, len - 1) x1[i] = x1[i] * x2[i]; ntt(x1, len, -1); vector <LL>().swap(c);
rep(i, 0, sz1 + sz2 - 2) c.push_back(x1[i]);
} int main(){ fac[0] = 1;
rep(i, 1, 1e5 + 3) fac[i] = fac[i - 1] * i % mod; scanf("%d", &T); while (T--){
scanf("%d", &n);
rep(i, 0, n + 1) vector <LL>().swap(v[i]); while (!q.empty()) q.pop(); all = 0; rep(i, 1, n){
int x, y;
scanf("%d%d", &x, &y);
v[i].resize(min(x, y) + 1);
rep(k, 0, min(x, y)) v[i][k] = C(x, k) * C(y, k) % mod * fac[k] % mod;
q.push(i);
all += x;
} cnt = n;
rep(i, 1, n - 1){
int x = q.top(); q.pop();
int y = q.top(); q.pop(); mul(v[x], v[y], v[++cnt]); vector <LL>().swap(v[x]);
vector <LL>().swap(v[y]); q.push(cnt);
} ans = 0;
flag = 1;
sz = (int)v[cnt].size();
rep(i, 0, sz - 1){
ans = ans + flag * fac[all - i] % mod * v[cnt][i] % mod;
ans = (ans + mod) % mod;
flag = -flag;
} printf("%lld\n", ans);
} return 0;
}