Text Editor Core LLD: The Gap Buffer and Undo That Knows Its Own Inverse

"Design the core of a text editor." Not the menus, not the syntax highlighting — the thing under the cursor that makes typing feel instant even in a 10,000-line file. This question is really two famous data-structure puzzles wearing one costume: how do you store text so cursor-local edits are cheap, and how do you undo? Get either backwards — a naïve string, an undo stack that snapshots the whole document — and you've built something that works in the demo and dies on a real file. The interviewer knows it; that's why they ask. The idea that unlocks both halves is almost philosophical: the expe…