Key-Value Store with Hit Counter

DatabricksOnsiteMedium

Design a string-to-string store that also tracks how often each key is read. Every read is stamped with the time it happened; times never decrease across calls, and several reads may share one time. Each test drives a single KVStore instance through a sequence of method calls and checks every return value.

Part 1: Put and get

Implement two methods.

put(key, value) stores value under key, replacing any earlier value, and returns "OK".
get(at, key) returns the value stored under key, or "NULL" if key was never stored. The empty string is a real value, so a key stored with "" returns "", not "NULL". at is the integer time of the read; in this part it does not affect the result.

Example 1
init
[]
operations
[["get",[1,"a"]],["put",["a","x"]],["get",[2,"a"]],["put",["a","y"]],["get",[2,"a"]],["put",["b",""]],["get",[3,"b"]],["get",[3,"c"]]]
Output
["NULL","OK","x","OK","y","OK","","NULL"]
Why: A get on a missing key returns NULL. After put(a, x) the next get returns x, and overwriting a with y makes the following get return y. An empty string is a real value, so get(b) returns it rather than NULL.

Constraints

1 <= len(key) <= 10
0 <= len(value) <= 10
'a' <= key[i] <= 'z'
1 <= len(operations) <= 10^4
at never decreases between calls to get