Fast Fourier transform &hdu 1402

Reference http://www.cnblogs.com/v-July-v/archive/2011/08/13/2214132.html

"Calculation Guide"

The faster polynomial multiplication, then, relies on the ability to quickly convert a polynomial of a coefficient form into a point-value pair, and the form of a point-value pair is quickly transformed into a coefficient form. such as the following form:

The evaluation + pointwise multiplication + interpolation three-hop process.

#include <iostream> #include <string.h> #include <stdio.h> #include <math.h>using namespace std    ; const int N = 500005;const Double PI = ACOs ( -1.0); struct virt{double R, I;        Virt (Double r = 0.0,double i = 0.0) {this->r = R;    This->i = i;    } Virt operator + (const Virt &x) {return Virt (R + X.R, i + x.i);    } Virt Operator-(const Virt &x) {return Virt (R-X.R, i-x.i);    } Virt operator * (const Virt &x) {return Virt (R * x.r-i * x.i, I * X.R + R * x.i);    }};//Reid algorithm--inverted-order void Rader (Virt f[], int len) {int j = Len >> 1;        for (int i=1; i<len-1; i++) {if (I < j) Swap (F[i], f[j]);        int k = Len >> 1;            while (J >= K) {J-= k;        K >>= 1;    } if (J < k) J + = k;    }}//fft implements a void FFT (Virt f[], int len, int on) {rader (F, Len); for (int h=2; h<=len; h<<=1)//divided by the calculation of the length of H DFT {VIRT wn (cos (-on*2*pi/h), sin (-on*2*pi/h));            Unit complex root e^ (2*pi/m) uses Euler formula to expand for (int j=0; j<len; j+=h) {Virt W (1,0);                Rotation factor for (int k=j; k<j+h/2; k++) {Virt u = f[k];                Virt t = w * f[k + H/2];     F[k] = U + t;                Butterfly merge operation F[k + H/2] = u-t;      w = w * WN; Update rotation Factor}} if (on = =-1) for (int i=0; i<len; i++) f[i].r/= Len;}    convolution void Conv (Virt a[],virt b[],int len) {FFT (a,len,1);    FFT (b,len,1);    for (int i=0; i<len; i++) a[i] = A[i]*b[i]; FFT (a,len,-1);} Char Str1[n],str2[n];    Virt va[n],vb[n];int result[n];int len;void Init (char Str1[],char str2[]) {int len1 = strlen (STR1);    int len2 = strlen (STR2);    len = 1;    while (Len < 2*len1 | | Len < 2*LEN2) Len <<= 1;    int i;        for (i=0; i<len1; i++) {VA[I].R = str1[len1-i-1]-' 0 ';    va[i].i = 0.0; } while (I < len) {VA[I].R = va[i].i = 0.0;    i++;        } for (i=0; i<len2; i++) {VB[I].R = str2[len2-i-1]-' 0 ';    vb[i].i = 0.0;        } while (I < len) {VB[I].R = vb[i].i = 0.0;    i++;    }}void work () {Conv (Va,vb,len); for (int i=0; i<len; i++) result[i] = va[i].r+0.5;}        void Export () {for (int i=0; i<len; i++) {result[i+1] + = RESULT[I]/10;    Result[i]%= 10;    } int high = 0;            for (int i=len-1, i>=0; i--) {if (Result[i]) {high = i;        Break    }} for (int i=high; i>=0; i--) printf ("%d", result[i]); Puts ("");}        int main () {while (~scanf ("%s%s", Str1,str2)) {Init (STR1,STR2);        Work ();    Export (); } return 0;}

　　

Fast Fourier transform &hdu 1402