NTT学习笔记
和\(FFT\)相对应的,把单位根换成了原根,把共轭复数换成了原根的逆元,最后输出的时候记得乘以原\(N\)的逆元即可.
#include <bits/stdc++.h>
using namespace std;
#define LL long long
const int MAXN = 3 * 1e6 + 10, P = 998244353, G = 3;
LL a[MAXN], b[MAXN];
int N, M, limit = 1, L, r[MAXN], Gi;
inline LL fastpow(LL a, LL k) {
LL base = 1;
while(k) {
if(k & 1) base = (base * a ) % P;
a = (a * a) % P;
k >>= 1;
}
return base % P;
}
inline void NTT(LL *A, int type) {
for (int i = 0; i < limit; i++) {
if(i < r[i]) swap(A[i], A[r[i]]);
}
for (int mid = 1; mid < limit; mid <<= 1) {
LL Wn = fastpow (type == 1 ? G : Gi , (P - 1) / (mid << 1));
for(int j = 0; j < limit; j += (mid << 1)) {
LL w = 1;
for (int k = 0; k < mid; k++, w = (w * Wn) % P) {
int x = A[j + k], y = (w * A[j + k + mid]) % P;
A[j + k] = (x + y) % P;
A[j + k + mid] = (x - y + P) % P;
}
}
}
}
int main () {
Gi = fastpow (G, P - 2);
cin >> N >> M;
for (int i = 0; i <= N; i++) {cin >> a[i]; a[i] = (a[i] + P) % P;}
for (int i = 0; i <= M; i++) {cin >> b[i]; b[i] = (b[i] + P) % P;}
while (limit <= N + M) limit <<= 1, L++;
for (int i = 0; i < limit; i++) r[i] = (r[i >> 1] >> 1) | ((i & 1) << (L - 1));
NTT (a, 1); NTT (b, 1);
for (int i = 0; i < limit; i++) a[i] = (a[i] * b[i]) % P;
NTT (a, -1);
LL inv = fastpow (limit, P - 2);
for (int i = 0; i <= N + M; i++) {
printf ("%d ", (a[i] * inv) % P);
}
return 0;
}