#175 · #631 · Design Excel Sum Formula(设计 Excel 求和公式)

Design a simplified Excel with set(row, column, val), get(row, column) and sum(row, column, numbers) where numbers is a list of cell references or ranges like "A1:B2". Columns are letters starting at A and rows start at 1. Sum cells must reflect later updates to their inputs. Constraints: 1 <= height <= 26, 1 <= width <= 26; at most 10^4 calls.