NTT 分类： templates 2015-08-11 23:34 3人阅读 评论(0) 收藏

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我觉得我还需要理解一下 FFT 和 NTT …

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#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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