Spreadsheets let each cell hold either a literal value or a formula that references other cells. In this question, implement a small Spreadsheet class with that behavior.

This question is intentionally limited:

• Cells only hold numbers or formula strings.
• Formula strings start with = and only use +.
• Referenced cells may themselves contain formulas.
• Inputs are guaranteed acyclic.

Examples

const sheet = new Spreadsheet();

sheet.setCell('A1', 10);
sheet.setCell('B1', '=A1 + 5');

sheet.getCell('B1'); // 15

Unset cells should behave like 0.

const sheet = new Spreadsheet();

sheet.setCell('A1', '=B1 + 3');

sheet.getCell('A1'); // 3

Referenced cells can contain formulas too.

const sheet = new Spreadsheet();

sheet.setCell('A1', 2);
sheet.setCell('B1', '=A1 + 3');
sheet.setCell('C1', '=B1 + 4');

sheet.getCell('C1'); // 9

Spreadsheet API

new Spreadsheet()

Creates a Spreadsheet instance with no cells.

spreadsheet.setCell(cellId, input)

Stores a value for cellId.

spreadsheet.getCell(cellId)

Returns the evaluated numeric value of cellId.

If cellId has not been set, return 0.

Notes

• Cell references use uppercase A1-style labels such as A1 and B12.
• Spaces inside formulas should be ignored.
• Formula operands are either cell references or unsigned numeric literals.
• You do not need subtraction, multiplication, division, cycles, ranges, or invalid-formula handling in this question.