What is the time complexity of accessing an element by key in a dictionary or hash table?

The average time complexity for accessing an element by key in a dictionary (hash table) is O(1) — constant time. This is achieved through computing the hash function of the key, which determines the index in the array.

However, in the worst case, due to collisions and poor distribution of keys, the complexity can degrade to O(n), where n is the number of elements. Modern implementations use collision resolution methods and table rehashing to minimize such cases.