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void fft(vector<ll>& a, bool inv = false) {
int n = ssize(a);
for (int s = 1; s < n; s *= 2) {
for (int i = 0; i < n; i += 2 * s) {
for (int j = i; j < i + s; j++) {
ll &u = a[j], &v = a[j + s];
v = inv ? v - u : v + u;
}}}}
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