Select table cells and set a color

Design a Table class for a whiteboard app. A table has a fixed number of rows and columns. A user selects part of the table and sets a color on everything selected.

Every cell starts with the empty color "". Only one selection is active at a time, and each select* call replaces the previous selection. Selecting does not change any color. Rows and columns are 0-indexed.

Implement:

• Table(numRows, numColumns): create the table. All cells have color "".
• selectTable(): select every cell.
• selectRow(row): select every cell in one row.
• selectCell(row, col): select one cell.
• setColor(color): set color on every selected cell. The selection stays active afterwards. If nothing has been selected yet, do nothing.
• getColor(row, col): return the current color of one cell.

The most recent setColor that covers a cell decides its color. An out-of-range row or column should raise an error.

Constraints

• 1 <= numRows, numColumns <= 10^6, so the table may have far too many cells to store one by one.
• Up to 10^5 calls in total.
• Aim for O(1) time per call (amortized is fine).

Example 1

t = Table(2, 3)
t.selectRow(0)
t.setColor("teal")
t.selectCell(0, 1)
t.setColor("gold")
t.getColor(0, 1)  -> "gold"
t.getColor(0, 2)  -> "teal"
t.getColor(1, 1)  -> ""

Example 2

t = Table(4, 4)
t.selectCell(3, 3)
t.setColor("navy")
t.selectTable()
t.setColor("white")
t.getColor(3, 3)  -> "white"
t.selectRow(2)
t.setColor("plum")
t.getColor(2, 0)  -> "plum"
t.getColor(3, 3)  -> "white"