递归FFT

#include <iostream>
#include <complex>
#include <cstdio>
#include <cmath>

const int mx_n = 4e5 + 10;
const double pi = acos(-1.0);
#define rep(i, s, t) for(register int i = s; i <= t; ++i)

using namespace std;

typedef complex<double> LF;
void DFT(LF *p, int n, int f) {
    if(n == 1) return ;
    n >>= 1;
    LF p0[n], p1[n];

    rep(i, 0, n-1) p0[i] = p[i<<1], p1[i] = p[i<<1|1];
    DFT(p0, n, f), DFT(p1, n, f);

    LF e(1, 0), w(cos(2*pi/(n<<1)), f*sin(2*pi/(n<<1)));
    rep(i, 0, n-1) {
        p[i] = p0[i] + e * p1[i];
        p[i+n] = p0[i] - e * p1[i];
        e = e * w;
    }
}

int n, m;
LF a[mx_n], b[mx_n], c[mx_n];
int main() {
#ifndef ONLINE_JUDGE
    freopen("input.in", "r", stdin);
    freopen("res.out", "w", stdout);
#endif

    scanf("%d%d", &n, &m);
    rep(i, 0, n) scanf("%lf", &a[i]);
    rep(i, 0, m) scanf("%lf", &b[i]);

    int N = 1;
    for(; N <= n + m; N <<= 1);
    DFT(a, N, 1), DFT(b, N, 1);

    rep(i, 0, N-1) c[i] = a[i] * b[i];
    DFT(c, N, -1);

    rep(i, 0, n + m) 
        printf("%d%c", (int)(c[i].real()/N + 0.5), i ^ (n+m)? ' ':'
');
    return 0;
}