Debug a Random Dasher Picker

Implement and debug a thread-safe random Dasher picker, RandomDasherPicker. The component maintains Dashers that are currently available for assignments and supports dynamic insertion, removal, and random selection. Implement: add(dasher_id): Add a Dasher. Do nothing if it already exists. remove(dasher_id): Remove a Dasher. Do nothing if it does not exist. pick(): Return a uniformly random current dasher_id; return null if no Dasher is available. The initial design stores Dashers in an index -> dasher_id mapping. To keep indices contiguous, when deleting a Dasher, move the last Dasher into the deleted index and then remove the old last index. Requirements: add, remove, and pick must each run in average O(1) time. No holes may remain in the index range after deletion. The component is called concurrently. It must not return a removed Dasher, throw concurrent-modification errors, or corrupt internal state. Do not invoke potentially blocking external APIs while holding the component's internal lock. Explain how a downstream call after pick should be structured. Example: add("A") add("B") add("C") remove("B") The resulting set is {"A", "C"}. A valid internal representation is: 0 -> "A" 1 -> "C" pick() may return only "A" or "C". Constraints: up to 10^5 Dashers and 10^6 operations. Example Input add A\nadd B\npick Output A 或 B