## Problem Build core operations for a browser-based paint editor using an HTML5 Canvas. Implement: freehand drawing (mouse drag), straight lines, flood-fill (paint bucket), and undo/redo with a 20-step history. ```typescript class PaintEditor { constructor(canvas: HTMLCanvasElement) {} setTool(tool: 'brush' | 'line' | 'fill'): void {} setColor(hex: string): void {} setBrushSize(px: number): void {} undo(): void {} redo(): void {} // Internal: called on mouse events private onMouseDown(e: MouseEvent): void {} private onMouseMove(e: MouseEvent): void {} private onMouseUp(e: MouseEvent): void {} } ``` **Example behavior:** ``` editor.setTool('fill'); editor.setColor('#FF0000'); // User clicks on a blue region -> flood-fill changes all connected blue pixels to red editor.undo(); // restores the blue region ``` ## Follow-ups 1. How does flood-fill work at the pixel level? What queue-based BFS approach do you use? 2. For undo/redo, do you store full canvas snapshots or operation deltas? What are the tradeoffs? 3. Freehand drawing at high mouse speed leaves gaps between points — how do you interpolate? 4. How would you add a selection tool with copy/paste and move operations?

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Paint Editor: Implement Core Canvas Drawing Operations for a Browser-Based Paint Tool

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Problem Build core operations for a browser-based paint editor using an HTML5 Canvas. Implement: freehand drawing (mouse drag), straight lines, flood-fill (paint bucket), and undo/redo with a 20-step history. Example behavior: Follow-ups How does flood-fill work at the pixel level? What queue-based BFS approach do you use? For undo/redo, do you store full canvas snapshots or operation deltas? What are the tradeoffs? Freehand drawing at high mouse speed leaves gaps between points — how do you int…

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About This Question

This is a candidate experience report from a merge interview during the phone round.

It covers the following topics: Strings, Phone, Graph, Coding, Queue .

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