又是我来debug了不好意思（

参考了b站董晓算法的代码，只用函数FFT做正逆变换就全AC过了。 但是如果改成FFT2做正变换会WA一个点；改成FFT2做逆变换更是只AC了两个点 /(ㄒoㄒ)/~~ 欢迎大佬指正

#include <stdio.h>
#include <stdlib.h>
#include <string.h> 
#include <complex.h>
#include <math.h>
#define M_PI 3.14159265358979323846

void FFT2(complex a[],int n,int b)
{
	int i,j,k;									//1.蝴蝶变换更快（不用递归 
	int *r=(int*)calloc(n,sizeof(int));				
	r[0]=0;
	for (i=0;i<n;i++) {
		r[i]=r[i/2]/2+(i&1)*n/2; 
//		printf("%d %d\n",i,r[i]);
	}
	for (i=0;i<n>>1;i++) {
		if (i<r[i]){
			int t=a[i];
			a[i]=a[r[i]];
			a[r[i]]=t;
		}
	}
//	for (i=0;i<n;i++) printf("%d %d\n",i,r[i]);		//测试 
	for (k=2;k<=n;k<<=1){
		complex w1=cos(2*M_PI/k)+I*sin(2*M_PI/k)*b;
		for (i=0;i<n;i+=k){
			complex wk=1;
			for (j=0;j<k/2;j++){
				complex x=a[i+j],y=a[i+j+k/2]*wk;
				a[i+j]=x+y;
				a[i+j+k/2]=x-y;
				wk*=w1;
			}
		}
	}
	free(r);
}
void FFT(complex a[],int n,int b)
{
	if (n==1) return;
	int i;	
	complex a1[n/2],a2[n/2];			//2.递归变换 
	for (i=0;i<n/2;i++){
		a1[i]=a[2*i];
		a2[i]=a[2*i+1];
	}
	FFT(a1,n/2,b);	FFT(a2,n/2,b);
	complex w1=cos(2*M_PI/n)+I*sin(2*M_PI/n)*b;
	complex wk=1;
	for (i=0;i<n/2;i++){
		a[i]=a1[i]+a2[i]*wk;
		a[i+n/2]=a1[i]-a2[i]*wk;
		wk*=w1;				
	} 
}
 
int main()
{ 
	complex a[5000000],b[5000000];
	int m,n,i;
	
	scanf("%d%d",&n,&m);						//最高项次数 
	for (i=0;i<=n;i++) scanf("%lf",&a[i]); 		//项数 = max次数 + 1  
	for (i=0;i<=m;i++) scanf("%lf",&b[i]);	
	for (m=m+n,n=1;n<=m;n<<=1);
	
//	printf("%d\n",n);	//测试 
	
	FFT2(a,n,1);	FFT2(b,n,1);
	for (i=0;i<n;i++) a[i]*=b[i];
	FFT(a,n,-1);						//这里用2会出错 
	
	for (i=0;i<=m;i++) printf("%d ",(int)(creal(a[i])/n+0.5));
	return 0;
}