Luogu P3803 【模板】多项式乘法（FFT）

代码中的变量 n 的意义是多项式的长度，也就是度数加一，而非度数。

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const int MOD = 998244353, G = 3;

int swp[MAXN + 10];
void EvaSwp(int n) {
    int mxdeg = n >> 1;
    for (int i = 1; i < n; ++i) {
        swp[i] = (swp[i >> 1] >> 1) + ((i & 1) ? mxdeg : 0);
    }
}
void Swp(int n, int *a) {
    for (int i = 0; i < n; ++i) {
        if (swp[i] > i) {
            swap(a[i], a[swp[i]]);
        }
    }
}
void NTT(int n, int *a, int getinv) {
    Swp(n, a);
    for (int len = 2, halflen = 1; len <= n; len <<= 1, halflen <<= 1) {
        int glen = power(G, (MOD - 1) / len);
        for (int l = 0, r = len - 1; r < n; l += len, r += len) {
            int g = 1;
            for (int i = l, j = l + halflen; j <= r; ++i, ++j, g = g * glen % MOD) {
                int u = a[i],
                    v = a[j] * g % MOD;
                a[i] = addmod(u + v);
                a[j] = redmod(u - v);
            }
        }
    }
    if (getinv == -1) {
        reverse(a + 1, a + n);
        int invn = inv(n);
        for (int i = 0; i < n; ++i) {
            a[i] = a[i] * invn % MOD;
        }
    }
}