All OOne Operations

A quick note: this problem is backed by a real Spotnana backend interview report that directly named the All O(1) Data Structure task. The report did not reproduce the complete API, examples, or constraints. FastPrep uses the standard inc, dec, getMaxKey, and getMinKey behavior in an operation-list interface. The core task match is about 95%.
Design a data structure that stores string counts and supports incrementing a key, decrementing a key, returning a key with the maximum count, and returning a key with the minimum count.
Process the given operations in order:
["inc", key]: increase key's count by 1, inserting it with count 1 if needed.
["dec", key]: decrease key's count by 1. If the count becomes 0, remove the key. The key exists before this operation.
["getMaxKey"]: append one key with the maximum count, or "" if the structure is empty.
["getMinKey"]: append one key with the minimum count, or "" if the structure is empty.
Return the outputs from the getMaxKey and getMinKey operations in order.
Interview Follow-up
When the implementation hit a segmentation fault, the candidate explained the doubly linked list and hash map design in detail. The interviewers said they were satisfied with the reasoning even without a fully compiling final submission.

Function
runAllOOne(operations: String[][]) → String[]

Examples
Example 1
operations = [["inc","hello"],["inc","hello"],["getMaxKey"],["getMinKey"],["inc","leet"],["getMaxKey"],["getMinKey"]]
return = ["hello","hello","hello","leet"]
After two increments, hello is both max and min. After leet is inserted once, hello remains max and leet is min.
Example 2
operations = [["getMaxKey"],["inc","a"],["inc","b"],["inc","b"],["dec","b"],["dec","b"],["getMinKey"],["getMaxKey"]]
return = ["","a","a"]
The first query sees an empty structure. Key b is removed after its count reaches zero, leaving only a.

The trick: keep buckets in a doubly linked list sorted by count, each bucket holding a set of keys with that count. A hash map sends key to its bucket node. Inc moves a key to the next bucket (count+1), creating it right after the current one if the count doesn't match. Dec moves it to the previous bucket, or deletes the key at zero. Max is the tail bucket, min is the head bucket. Use sentinel head and tail nodes so you never null-check edges. The common pitfall is forgetting to delete an empty bucket after moving a key out, which makes getMin return a stale or empty set. Also handle the empty structure by returning an empty string. Link updates must happen in the right order or you get the segfault. If the pointer logic slips under pressure, StealthCoder can give you a working reference while the live OA runs.

StealthCoder is the hedge for the one pattern you didn't drill. It runs invisibly during the screen share.