模板

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

const int MAXN = 4e6;
const double PI = acos(-1.0);

struct Complex {
    double x, y;
    Complex() {}
    Complex(double x, double y): x(x), y(y) {}
    friend Complex operator+(const Complex &a, const Complex &b) {
        return Complex(a.x + b.x, a.y + b.y);
    }
    friend Complex operator-(const Complex &a, const Complex &b) {
        return Complex(a.x - b.x, a.y - b.y);
    }
    friend Complex operator*(const Complex &a, const Complex &b) {
        return Complex(a.x * b.x - a.y * b.y, a.x * b.y + a.y * b.x);
    }
} A[MAXN + 5], B[MAXN + 5];

void FFT(Complex a[], int n, int op) {
    for(int i = 1, j = n >> 1; i < n - 1; ++i) {
        if(i < j)
            swap(a[i], a[j]);
        int k = n >> 1;
        while(k <= j) {
            j -= k;
            k >>= 1;
        }
        j += k;
    }
    for(int len = 2; len <= n; len <<= 1) {
        Complex wn(cos(2.0 * PI / len), sin(2.0 * PI / len)*op);
        for(int i = 0; i < n; i += len) {
            Complex w(1.0, 0.0);
            for(int j = i; j < i + (len >> 1); ++j) {
                Complex u = a[j], t = a[j + (len >> 1)] * w ;
                a[j] = u + t, a[j + (len >> 1)] = u - t;
                w = w * wn;
            }
        }
    }
    if(op == -1) {
        for(int i = 0; i < n; ++i)
            a[i].x = (int)(a[i].x / n + 0.5);
    }
}

int pow2(int x) {
    int res = 1;
    while(res < x)
        res <<= 1;
    return res;
}

void convolution(Complex A[], Complex B[], int Asize, int Bsize) {
    int n = pow2(Asize + Bsize - 1);
    for(int i = 0; i < n; ++i) {
        A[i].y = 0.0;
        B[i].y = 0.0;
    }
    for(int i = Asize; i < n; ++i)
        A[i].x = 0;
    for(int i = Bsize; i < n; ++i)
        B[i].x = 0;
    FFT(A, n, 1);
    FFT(B, n, 1);
    for(int i = 0; i < n; ++i)
        A[i] = A[i] * B[i];
    FFT(A, n, -1);
    return;
}

int main() {
#ifdef Yinku
    freopen("Yinku.in", "r", stdin);
#endif // Yinku
    int n, m;
    scanf("%d%d", &n, &m);
    for(int i = 0; i <= n; ++i) {
        scanf("%lf", &A[i].x);
    }
    for(int i = 0; i <= m; ++i) {
        scanf("%lf", &B[i].x);
    }
    convolution(A, B, n + 1, m + 1);
    for(int i = 0; i <= n + m; i++) {
        printf("%d%c", (int)A[i].x, " 
"[i == n + m]);
    }
    return 0;
}

有一个精度更高（在多次调用时速度也更快）的版本，需要预处理单位根，花费多一点空间：

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

const double PI = acos(-1.0);
const int MAXN = 1e6;

struct Complex {
    double x, y;
    Complex(): x(0), y(0) {}
    Complex(double x, double y): x(x), y(y) {}
    friend Complex operator+(const Complex &a, const Complex &b) {
        return Complex(a.x + b.x, a.y + b.y);
    }
    friend Complex operator-(const Complex &a, const Complex &b) {
        return Complex(a.x - b.x, a.y - b.y);
    }
    friend Complex operator*(const Complex &a, const Complex &b) {
        return Complex(a.x * b.x - a.y * b.y, a.x * b.y + a.y * b.x);
    }
};

typedef vector<Complex> Poly;

Poly w[2 * MAXN + 5][2];
int rev[4 * MAXN + 5];

inline void FFT(Poly &a, int n, int op) {
    for(int i = 0; i < n ; ++i) {
        if(i < rev[i])
            swap(a[i], a[rev[i]]);
    }
    for(int len = 2; len <= n; len <<= 1) {
        register int m = len >> 1;
        for(int i = 0; i < m; ++i) {
            Complex &tw = w[m][op == 1][i];
            for(int j = i; j < n; j += len) {
                Complex u = a[j], t = a[j + m] * tw ;
                a[j] = u + t, a[j + m] = u - t;
            }
        }
    }
    if(op == -1) {
        for(int i = 0; i < n; ++i) {
            a[i].x = (ll)(a[i].x / n + 0.5);
            a[i].y = 0;
        }
    }
}

inline int pow2(int x, int &lgn) {
    int res = 1;
    lgn = 0;
    while(res < x) {
        if(!w[res][0].size()) {
            w[res][0].resize(res);
            w[res][1].resize(res);
            for(int i = 0; i < res; ++i) {
                //0是逆变换需要的
                w[res][0][i] = Complex(cos(-PI * i  / res), sin(-PI * i / res));
                w[res][1][i] = Complex(cos(PI * i  / res), sin(PI * i  / res));
            }
        }
        res <<= 1;
        ++lgn;
    }
    return res;
}

inline int convolution(Poly &A, Poly &B, int Asize, int Bsize) {
    int lgn, n = pow2(Asize + Bsize - 1, lgn);
    for(int i = 0; i < n; ++i)
        rev[i] = (rev[i >> 1] >> 1) | ((i & 1) << lgn - 1);
    A.resize(n);
    B.resize(n);
    FFT(A, n, 1);
    FFT(B, n, 1);
    for(int i = 0; i < n; ++i)
        A[i] = A[i] * B[i];
    FFT(A, n, -1);
    while(n && (A[n - 1].x == 0))
        --n;
    return n;
}

Poly poly[MAXN + 5];


int main() {
#ifdef Yinku
    freopen("Yinku.in", "r", stdin);
#endif // Yinku
    int n, m;
    scanf("%d%d", &n, &m);
    Poly A, B;
    for(int i = 0; i <= n; ++i) {
        int x;
        scanf("%d", &x);
        A.push_back(Complex(x, 0));
    }
    for(int i = 0; i <= m; ++i) {
        int x;
        scanf("%d", &x);
        B.push_back(Complex(x, 0));
    }

    int C = convolution(A, B, n + 1, m + 1);
    for(int i = 0; i <= n + m; ++i) {
        printf("%lld%c", (ll)A[i].x, " 
"[i == n + m]);
    }
    return 0;
}