Methods

1) void addText(int row, int column, String text)

• Insert text into the specified row at position column.
• Existing characters at/after column shift right.
• Input rules:
  • text contains only: letters [a-z], [A-Z] , space ' ', hyphen -.
  • 0 ≤ row ≤ current row count.
  • If row == current row count, first append a new empty row, then insert (you cannot skip rows).
  • 0 ≤ column ≤ current column count of the target row (after creating it if new).
  • text.length ≥ 1.
• Effects: only the targeted row changes.
• This operation pushes one entry onto the Undo history.
• After any successful addText, the Redo history is cleared.

2) void deleteText(int row, int startColumn, int length)

• Delete exactly length characters from row, starting at startColumn.
• Input rules:
  • 0 ≤ row < current row count.
  • 1 ≤ length.
  • 0 ≤ startColumn + length ≤ current column count of that row.
• Effects: only the targeted row changes, empty rows remain (row count does not decrease).
• This operation pushes one entry onto the Undo history.
• After any successful deleteText, the Redo history is cleared.

3) void undo()

• Revert the most recent change (addText or deleteText) that has not yet been undone.
• If there is no change to undo, undo() is a no-op.
• The reverted change is pushed onto the Redo stack.

4) void redo()

• Reapply the most recently undone change.
• If no undone change exists, redo() is a no-op.
• After any new edit (addText or deleteText) since the last undo(), the Redo stack is cleared and redo() will have no effect until another undo().
• The reapplied change is pushed back onto the Undo stack.

5) String readLine(int row)

• Return the entire content of the specified row.
• Rules: 0 ≤ row < current row count.
• Returns "" if the row exists but is currently empty.

Assumptions

• All inputs respect constraints (no need to handle invalid indices).
• Document may contain empty rows; these are valid and readable ("").
• No trimming or normalization; spaces and hyphens are preserved exactly.

Examples

Assume the editor starts with 0 rows.

1. Example 1 - Creating Rows and Appending Text
   • addText(0, 0, "hello") → creates row 0 with hello
   • readLine(0) → hello
   • addText(1, 0, "world") → appends new row 1 with world
   • readLine(1) → world
2. Example 2 - Insert in Middle, Delete, Undo/Redo
   • (Starting rows) 0: "hello, 1: "world
   • addText(0, 5, "-there") → row 0 becomes hello-there
   • readLine(0) → hello-there
   • deleteText(0, 5, 6) → delete -there, row 0 becomes hello
   • readLine(0) → hello
   • undo() → restore -there, row 0: hello-there
   • readLine(0) → hello-there
   • redo() → re-apply delete, row 0: hello
   • readLine(0) → hello
3. Example 3 - Insert into Middle of Line
   • (Start) row 1 = world
   • addText(1, 5, "-wide web") → row 1 becomes world-wide web
   • readLine(1) → world-wide web
   • deleteText(1, 5, 5) → delete -wide, row 1: world web
   • undo() → restore -wide, row 1: world-wide web
   • readLine(1) → world-wide web
4. Example 4 - Multiple Undos, Redo Cleared by New Edit
   • (Start) row 0 = hello
   • addText(0, 5, "!") → hello!
   • addText(0, 6, "!") → hello!!
   • undo() → revert second add → hello!
   • undo() → revert first add → hello
   • redo() → reapply first add → hello!
   • addText(0, 6, "?") → hello!? (clears Redo)
   • redo() → no-op (Redo is empty)
   • readLine(0) → hello!?
5. Example 5 - Deleting From Start, Empty Row Persists
   • addText(0, 0, "aa bb-cc") → row 0: aa bb-cc
   • deleteText(0, 0, 8) → delete entire content, row 0:
   • readLine(0) →
   • undo() → restore aa bb-cc
   • readLine(0) → aa bb-cc
6. Example 6 - Inserting at Column 0 (Prefix Insert)
   • addText(1, 0, "world") → row 1: world
   • addText(1, 0, "hello-") → insert at start, row 1: hello-world
   • readLine(1) → hello-world