Design time-based key-value store

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"