萌新刚学OI，求神佬帮助

#include<iostream>
#include<cstdio>
#include<cmath>
#include<complex>
#include<algorithm>
#define PI acos(-1)
using namespace std;

const int MAXN = 2097152 + 5;

typedef complex <double> cp;

cp w(int n , int k) {return cp(cos(k / n * 2 * PI) , sin(k / n * 2 * PI));}
void fft(cp* a , int n , bool inv) { //[a , a + n)
	if(n == 1) return ;
	cp buf[MAXN]; int m = n / 2;
	for(int i = 0 ; i < m ; ++i) buf[i] = a[i * 2] , buf[i + m] = a[i * 2 + 1];
	for(int i = 0 ; i < n ; ++i) a[i] = buf[i];
	fft(a , m , inv); fft(a + m , m , inv);
	for(int i = 0 ; i < m ; ++i) {
		cp o = w(n , i); if(inv) o = conj(o);
		buf[i] = a[i] + a[i + m] * o , buf[i + m] = a[i] - a[i + m] * o;
	}
	for(int i = 0 ; i < n ; ++i) a[i] = buf[i];
}

int n , m , A[MAXN] , B[MAXN] , C[MAXN] , lg[MAXN];
cp a[MAXN] , b[MAXN] , c[MAXN];

int main() {
	scanf("%d %d" , &n , &m); ++n , ++m;
	for(int i = 2 ; i < MAXN ; ++i) lg[i] = lg[i / 2] + 1;
	for(int i = 0 ; i < n ; ++i) scanf("%d" , A + i);
	for(int i = 0 ; i < m ; ++i) scanf("%d" , B + i);
	int g = max(n , m) << 1; n = m = (1 << (lg[g] + 1)); 
	for(int i = 0 ; i < n ; ++i) a[i] = cp(A[i] , 0) , b[i] = cp(B[i] , 0);
	fft(a , n , 0); fft(b , m , 0);
	for(int i = 0 ; i < n ; ++i) c[i] = a[i] * b[i];
	fft(c , n , 1);
	for(int i = 0 ; i < n ; ++i) printf("%d " , (int)(real(c[i]) / n + 0.5));
	return 0;
}