题解 P5050 【【模板】多项式多点求值】

Memory_of_winter

2018-12-31 13:25:34

Solution

[我的博客](https://www.cnblogs.com/Memory-of-winter/p/10201900.html) **题目大意:**给你一个$n$次多项式$f(x)$,以及$m$个$x_i$,对于$i\in[1,m]$,求$f(x_i)$ **题解:**多项式多点求值 令$g(x)=\prod\limits_{i=1}^m(x-x_i)$,求出$R(x)$使得$f(x)=Q(x)\times g(x)+R(x)$。因为当$x=x_i$时,$g(x)=0$,即$f(x)=R(x)$,$f(x)$是$n$次的,$R(x)$是$m-1$次的,似乎可以使得问题缩小了 考虑分治,现在区间为$[l,r]$,令$g_L(x)=\prod\limits_{i=l}^{mid}(x-x_i)$,$g_R(x)=\prod\limits_{i=mid}^r(x-x_i)$,所以$R_L(x)=f(x)\bmod g_L(x)$,$R_R(x)=f(x)\bmod g_R(x)$。最后当$l=r$时,第$i$个的值就是当前$R(x)$的常数项。 那$g(x)$怎么算呢,分治$FFT$,可以先把每个的$g(x)$求出来,用$vector$保存一下就行了 **卡点:**不知道为什么,用$C++$会$MLE$,$C++11$就过了,有可能是$vector$初始化部分出锅了 **C++ Code:** ```cpp #include <cstdio> #include <algorithm> #include <vector> const int mod = 998244353, G = 3; namespace Math { inline int pw(int base, int p) { static int res; for (res = 1; p; p >>= 1, base = static_cast<long long> (base) * base % mod) if (p & 1) res = static_cast<long long> (res) * base % mod; return res; } inline int inv(int x) { return pw(x, mod - 2); } } inline void reduce(int &x) { x += x >> 31 & mod; } #define maxn 65536 int a[maxn], ans[maxn]; namespace Poly { #define N maxn int rev[N], lim, s, ilim; int Wn[N + 1]; inline void clear(register int *l, const int *r) { if (l >= r) return ; while (l != r) *l++ = 0; } inline void init(const int n) { s = -1, lim = 1; while (lim < n) lim <<= 1, ++s; ilim = Math::inv(lim); for (register int i = 0; i < lim; ++i) rev[i] = rev[i >> 1] >> 1 | (i & 1) << s; const int t = Math::pw(G, (mod - 1) / lim); *Wn = 1; for (register int *i = Wn; i != Wn + lim; ++i) *(i + 1) = static_cast<long long> (*i) * t % mod; } inline void NTT(int *A, const int op = 1) { static int Wt[N]; for (register int i = 1; i < lim; ++i) if (i < rev[i]) std::swap(A[i], A[rev[i]]); for (register int mid = 1; mid < lim; mid <<= 1) { const int t = lim / mid >> 1; *Wt = Wn[op ? 0 : lim]; for (register int *i = Wt, W = 0; i != Wt + mid; ++i, W += t) *i = Wn[op ? W : lim - W]; for (register int i = 0; i < lim; i += mid << 1) { for (register int j = 0; j < mid; ++j) { const int X = A[i + j], Y = static_cast<long long> (Wt[j]) * A[i + j + mid] % mod; reduce(A[i + j] += Y - mod), reduce(A[i + j + mid] = X - Y); } } } if (!op) for (register int *i = A; i != A + lim; ++i) *i = static_cast<long long> (*i) * ilim % mod; } std::vector<int> P[N << 1], S[N << 1]; int C[N], D[N]; void DC_NTT(int rt, int l, int r) { if (l == r) { P[rt] = {a[l], 1}; return ; } int mid = l + r >> 1; DC_NTT(rt << 1, l, mid), DC_NTT(rt << 1 | 1, mid + 1, r); int L = rt << 1, R = rt << 1 | 1; int n = P[L].size(), m = P[R].size(); init(n + m - 1); std::copy(P[L].begin(), P[L].end(), C); clear(C + n, C + lim); std::copy(P[R].begin(), P[R].end(), D); clear(D + m, D + lim); NTT(C), NTT(D); for (int i = 0; i < lim; ++i) C[i] = static_cast<long long> (C[i]) * D[i] % mod; NTT(C, 0); P[rt].assign(C, C + n + m - 1); } int E[N]; void INV(int *A, int *B, int n) { if (n == 1) { *B = Math::inv(*A); return ; } INV(A, B, n + 1 >> 1); init(n + n - 1); std::copy(A, A + n, E); clear(E + n, E + lim); clear(B + (n + 1 >> 1), B + lim); NTT(B), NTT(E); for (int i = 0; i < lim; ++i) B[i] = (2 + mod - static_cast<long long> (B[i]) * E[i] % mod) * B[i] % mod; NTT(B, 0); clear(B + n, B + lim); } int F[N]; void DIV(int A, int n, int B, int m) { const int len = n - m + 1; init(len << 1); std::reverse_copy(S[A].begin(), S[A].end(), C); clear(C + len, C + lim); std::reverse_copy(P[B].begin(), P[B].end(), D); clear(D + len, D + lim); clear(F, F + lim); INV(D, F, len); NTT(C), NTT(F); for (int i = 0; i < lim; ++i) F[i] = static_cast<long long> (F[i]) * C[i] % mod; NTT(F, 0); clear(F + len, F + lim); } void __DIVMOD(int res, int A, int n, int B, int m) { if (n < m) { S[res].assign(S[A].begin(), S[A].end()); return ; } DIV(A, n, B, m); init(n); std::reverse_copy(F, F + n - m + 1, C); clear(C + n - m + 1, C + lim); std::copy(P[B].begin(), P[B].end(), D); clear(D + m, D + lim); NTT(C), NTT(D); for (int i = 0; i < lim; ++i) C[i] = static_cast<long long> (C[i]) * D[i] % mod; NTT(C, 0); for (int i = 0; i < m - 1; ++i) reduce(C[i] = S[A][i] - C[i]); S[res].assign(C, C + m - 1); } void DIVMOD(int res, int A) { int n = S[A].size(), m = P[res].size(); __DIVMOD(res, A, n, res, m); } void solve(int rt, int l, int r) { if (l == r) { ans[l] = S[rt][0]; return ; } int mid = l + r >> 1; DIVMOD(rt << 1, rt), DIVMOD(rt << 1 | 1, rt); solve(rt << 1, l, mid), solve(rt << 1 | 1, mid + 1, r); } void work(int *f, int n, int m) { DC_NTT(1, 1, m); S[0].assign(f, f + n); DIVMOD(1, 0); solve(1, 1, m); } #undef N } int n, m; int f[maxn]; int main() { scanf("%d%d", &n, &m); if (!m) return 0; ++n; for (int i = 0; i < n; ++i) scanf("%d", f + i); for (int i = 1; i <= m; ++i) scanf("%d", a + i), reduce(a[i] = -a[i]); Poly::work(f, n, m); for (int i = 1; i <= m; ++i) printf("%d\n", ans[i]); return 0; } ```