Problem
You are given two vectors A and B, for each 1 <= k <= 2*N we want to find out C[k] where C[k] is sum of all A[i]*B[j] such that i + j == k.
Constraints
N <= 1e6
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Auto comment: topic has been updated by Misa-Misa (previous revision, new revision, compare).
its just FFT bro