1. Why are there missing keys with 2 threads, but not with 1 thread?
    With 1 thread, the put and insert are always interleaved.
    No changes to hash table happen at the same time.
    But with 2 threads, there will be race to access the hash table,
    and this leads to the missing keys.

2. Identify a sequence of events with 2 threads that can lead to a key being missing.
    Suppose thread No. 1 is running the insert and so does thread No. 2.
    They shall both insert a entry. But because they enter the insert function with the same hash table,
    they could only insert one key at most, which causes a key being missing.