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void fft(vector<cplx>& a, bool inverse = 0) {
int n = sz(a);
for (int i = 0, j = 1; j < n - 1; ++j) {
for (int k = n >> 1; k > (i ^= k); k >>= 1);
if (j < i) swap(a[i], a[j]);
}
for (int s = 1; s < n; s *= 2) {
for (int j = 0; j < n; j+= 2 * s) {
for (int k = 0; k < s; k++) {
ll u = a[j + k], t = a[j + s + k];
if (!inverse) {
a[j + k] = t;
a[j + s + k] = u + t;
} else {
a[j + k] = t - u;
a[j + s + k] = u;
}}}}}
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