Design a Text Editor

Design a Text Editor

1. What is this problem about?

The Design a Text Editor interview question involves building a simulation of a basic text editor. You need to support inserting text at a cursor, deleting text before the cursor, and moving the cursor left or right. When moving, you also need to return a snippet of the text surrounding the cursor. This Design a Text Editor coding problem is all about efficient string and pointer manipulation.

2. Why is this asked in interviews?

Companies like Uber and Meta ask this to evaluate your ability to handle Dynamic Data Streams. It tests your knowledge of Linked List interview patterns or Stack interview patterns for representing a "gap" or a "cursor." It evaluates how you handle large strings that undergo frequent modifications at an arbitrary midpoint.

3. Algorithmic pattern used

The most efficient approach is the Two-Stack Pattern (or a Doubly-Linked List with a cursor pointer).

• Left Stack: Stores all characters to the left of the cursor.
• Right Stack: Stores all characters to the right of the cursor.
• Insert: Push characters to the Left Stack.
• Delete: Pop from the Left Stack.
• Move Left: Pop from the Left Stack and push to the Right Stack.
• Move Right: Pop from the Right Stack and push to the Left Stack. This makes all operations $O( ext{Length of change})$ rather than $O( ext{Total string length})$.

4. Example explanation

Initial: | (Left Stack: [], Right Stack: [])

1. addText("hello"): hello| (Left: [h,e,l,l,o], Right: [])
2. cursorLeft(2): hel|lo (Pop 'o', 'l' from Left, push to Right. Left: [h,e,l], Right: [o,l])
3. deleteText(1): he|lo (Pop 'l' from Left. Left: [h,e], Right: [o,l])
4. cursorRight(1): hel|o (Pop 'l' from Right, push to Left. Left: [h,e,l], Right: [o])

5. Common mistakes candidates make

• Using a Single String: Using standard string concatenation or substring() for every operation, which is $O(N)$ and extremely slow for large texts.
• Complexity Overload: Trying to use a Rope data structure or a Segment Tree when two simple stacks or a linked list are much easier to implement and sufficiently fast.
• Off-by-one snippet: Returning the wrong number of characters when querying the text to the left of the cursor.

6. Interview preparation tip

Whenever you have a "Cursor" that moves back and forth in a sequence, think of the Two-Stack approach. It’s also the secret to solving the "Undo/Redo" logic in many software design questions.