NTT

来源：互联网发布：淘宝没有扣分的违规编辑：程序博客网时间：2024/06/07 05:46

我觉得我还需要理解一下 FFT 和 NTT …

#include <cstdio>#include <cstdlib>#include <iostream>#include <algorithm>const int maxn = 4e4+5, Mod = 950009857;const int G = 7;int n, m, k, N = 1;long long g[maxn], invN;long long f[maxn], c[maxn];long long p[maxn];long long power(long long x,int t){    long long ret = 1;    while(t)    {        if(t&1) ret *= x, ret %= Mod;        x *= x, x %= Mod, t >>= 1;    }    return ret;}void PreWork(){    while(N <= (n<<1)) N <<= 1;    invN = power(N, Mod - 2);    g[0] = g[N] = 1, g[1] = power(G, (Mod-1)/N);     for(int i = 2; i < N; i++)        g[i] = g[i-1] * g[1] % Mod;}void NTT(long long a[],int len,int flag){    for(int i = 0; i < len; i++)    {        int s = i, p = 0;        for(int j = len>>1; j > 0; j >>= 1)            p |= (s&1)*j, s >>= 1;        if(p > i) std::swap(a[p], a[i]);    }    for(int L = 2 ; L <= len; L <<= 1)        for(int i = 0; i < len ; i += L)            for(int j = 0; j < (L>>1); j++)            {                long long w = (flag == 1)?g[N/L*j]:g[N - N/L*j];                long long u = a[i + j], v = a[i + j +(L>>1)]*w % Mod;                u += v, v = u - (v<<1), a[i + j] = u % Mod;                a[i + j +(L>>1)] = (v + Mod) % Mod;            }    if(flag == -1)        for(int i = 0; i < len; i++)            a[i] = a[i] * invN % Mod;}

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