The problem is the following:
We have array $$$a$$$ of $$$n$$$ integers and for each integer $$$k$$$ from $$$1$$$ to $$$n$$$ we want to calculate the number of pairs $$$(i, j)$$$ such that $$$a_i + a_j = k$$$.
I wonder if it can be solved in less than $$$\mathcal{O}(n^2)$$$
UPD: turns out it is a classical FFT problem