How much additional memory does a solution with a dictionary require, excluding the returned data?

Using a dictionary (hash table) requires additional memory proportional to the number of unique keys added to it. If you have N unique elements, the memory will be approximately O(N), since space is allocated for each key and its associated value.

For example, if you create a dictionary to count the occurrences of words in a text, the memory will grow with the number of unique words. The dictionary structure itself usually includes additional overhead for storing hash values and managing collisions, but overall, an O(N) estimate for additional memory is the most practical.