Design an in-memory key-value store that supports time-based lookups. ## Operations Implement a class (or module) with the following methods: 1. `set(key, value, timestamp)` - Stores the string `value` for string `key` at integer `timestamp`. - Multiple values may be stored for the same key at different timestamps. 2. `get(key, timestamp)` - Returns the value associated with `key` for the **largest stored timestamp `t` such that `t <= timestamp`**. - If there is no such timestamp for the key, return an empty string (or `null`, but be consistent). ## Notes / Constraints - Timestamps for `set()` calls on the same key are non-decreasing. - Aim for efficient `get()` queries. - You may assume ASCII strings for keys/values. ## Example - `set("room", "A", 10)` - `set("room", "B", 20)` - `get("room", 5) -> ""` - `get("room", 10) -> "A"` - `get("room", 15) -> "A"` - `get("room", 20) -> "B"` - `get("room", 25) -> "B"`