HackerRank CodePair HLD: Two People, One File, No Conflicts

"Design a collaborative code editor" — HackerRank CodePair, a Google-Docs-style document, a shared interview whiteboard. Two people type into the same file at the same time, and the screens must never disagree. That's the entire challenge, and it's deceptively deep: when both type a character "in the same place" at the same instant, whose goes first — and how does every screen agree on the answer without a central referee adding latency to every keystroke? The naïve answer — "insert at index 5" — falls apart immediately: by the time your "insert at 5" reaches me, my own edit has shifted everyt…