Implement an iOS app that displays a 2D grid of cells colored with two colors. When the user taps a cell, all 4-directionally adjacent cells of the same original color should be "infected" and change to the tapped color in a single interaction. Describe your UI architecture (UIKit or SwiftUI), data structures to represent the grid, and the flood-fill algorithm (iterative vs. recursive) with time/space complexity. Handle large grids efficiently, avoid stack overflow, and support reset/undo.

This question evaluates proficiency in mobile UI architecture, grid data structures, connected-component algorithm reasoning, and state management concerns such as undo and memory/stack safety within the mobile app development and iOS domain.

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This is a medium difficulty Other / Miscellaneous question, commonly asked during Onsite rounds at Pinterest.

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Implement tap-to-infect color grid on iOS | Pinterest Interview Question

iOS Grid Infection (Flood Fill) Design

Goal

Build an iOS app that displays a 2D grid with two colors. When the user taps a cell, all 4-directionally adjacent cells in the same connected component as the tapped cell should be flood-filled in a single interaction.

Assumptions (to make the problem well-scoped)

The grid contains exactly two colors (e.g., Color A and Color B).
On tap at cell (r, c), we flood-fill the connected component of the tapped cell’s original color to the other color (toggle). Alternatively, if a current paint color is selected in UI, fill to that color; the algorithm and design remain the same.
Adjacency is 4-directional (up, down, left, right).
The app should handle large grids efficiently, avoid stack overflow, and support reset and undo.

Deliverables

Describe:

UI architecture (UIKit or SwiftUI) and view structure.
Data structures to represent the grid.
Flood-fill algorithm (iterative vs. recursive) with time/space complexity.
How to handle large grids efficiently and avoid stack overflow.
How to support reset and undo.

Assumptions (to make the problem well-scoped)

The grid contains exactly two colors (e.g., Color A and Color B).

On tap at cell (r, c), we flood-fill the connected component of the tapped cell’s original color to the other color (toggle). Alternatively, if a current paint color is selected in UI, fill to that color; the algorithm and design remain the same.

Adjacency is 4-directional (up, down, left, right).

The app should handle large grids efficiently, avoid stack overflow, and support reset and undo.

Implement tap-to-infect color grid on iOS

Quick Overview

iOS Grid Infection (Flood Fill) Design

Goal

Assumptions (to make the problem well-scoped)

Deliverables

Solution

Submit Your Answer to Earn 20XP

Implement tap-to-infect color grid on iOS

Quick Overview

iOS Grid Infection (Flood Fill) Design

Goal

Assumptions (to make the problem well-scoped)

Deliverables

Solution

Submit Your Answer to Earn 20XP