System Design — Time-Based Key-Value Store

A versioned store: every write keeps its history, and a read asks "what was the value at
time T?".

solve(args, ops) builds new TimeMap() and replays ops:

• set(key, value, timestamp) — store value for key at timestamp. For a given key,

timestamps are strictly increasing across calls. Returns nothing.

• get(key, timestamp) — return the value stored at the **largest timestamp

<= timestamp** for that key, or "" if there is none (unknown key, or every write is in
the future).

Because the timestamps of a key arrive sorted, each key's history is a sorted array and
get is a binary search (the same "last element <= target" search as in the Searching
path), O(log n).

Example

set("foo","bar",1)   get("foo",1) → "bar"    get("foo",3) → "bar"
set("foo","bar2",4)  get("foo",4) → "bar2"   get("foo",0) → ""