FFT

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FFT新手请参考下面链接

http://www.gatevin.moe/acm/fft算法学习笔记/

#include<cstdio>#include<cstring>#include<iostream>#include<algorithm>#include<cmath>using namespace std;const int maxn=220005;const double eps(1e-8);typedef long long LL;const double PI = acos(-1.0);struct Complex{    double real, image;    Complex(double _real, double _image)    {        real = _real;        image = _image;    }    Complex(){}};Complex operator + (const Complex &c1, const Complex &c2){    return Complex(c1.real + c2.real, c1.image + c2.image);}Complex operator - (const Complex &c1, const Complex &c2){    return Complex(c1.real - c2.real, c1.image - c2.image);}Complex operator * (const Complex &c1, const Complex &c2){    return Complex(c1.real*c2.real - c1.image*c2.image, c1.real*c2.image + c1.image*c2.real);}int rev(int id, int len){    int ret = 0;    for(int i = 0; (1 << i) < len; i++)    {        ret <<= 1;        if(id & (1 << i)) ret |= 1;    }    return ret;}Complex A[maxn<<1];void FFT(Complex* a, int len, int DFT)//对a进行DFT或者逆DFT, 结果存在a当中,len必须是2的幂而且len大于多项式最高次数{    for(int i = 0; i < len; i++)        A[rev(i, len)] = a[i];    for(int s = 1; (1 << s) <= len; s++)    {        int m = (1 << s);        Complex wm = Complex(cos(DFT*2*PI/m), sin(DFT*2*PI/m));        for(int k = 0; k < len; k += m)        {            Complex w = Complex(1, 0);            for(int j = 0; j < (m >> 1); j++)            {                Complex t = w*A[k + j + (m >> 1)];                Complex u = A[k + j];                A[k + j] = u + t;                A[k + j + (m >> 1)] = u - t;                w = w*wm;            }        }    }    if(DFT == -1) for(int i = 0; i < len; i++) A[i].real /= len, A[i].image /= len;    for(int i = 0; i < len; i++) a[i] = A[i];    return;}Complex a[maxn<<1];int san[maxn];int num[maxn<<1];LL xishu[maxn<<1];int main(){    int t,n;    cin>>t;    while(t--){        scanf("%d",&n);        int len=1;        int s=0;        for(int i=1;i<=n;i++){            scanf("%d",&san[i]);            s=max(s,san[i]);        }        while(len<=s){            len<<=1;        }        len<<=1;        sort(san+1,san+1+n);        for(int i=0;i<len;i++){            a[i]=Complex(0,0);            num[i]=0;        }        for(int i=1;i<=n;i++){            a[san[i]].real+=1;            num[san[i]]++;        }        FFT(a,len,1);        for(int i=0;i<len;i++){            a[i]=a[i]*a[i];        }        FFT(a,len,-1);    }}

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