Difference of Number of Distinct Values on Diagonals

For each cell in an m×n grid, compute answer[r][c] = |(# distinct values strictly on the diagonal above‑left of (r,c)) − (# distinct values strictly on the diagonal below‑right of (r,c))|. The key is to process each bottom‑right diagonal independently and derive per‑cell prefix/suffix distinct counts to fill the answer matrix efficiently.