Find top-K frequent elements in a stream

You receive a large stream of items (e.g., integers or strings) that may not fit in memory. Design an algorithm that can return the top K most frequent items.

Requirements:

• Provide an exact solution when the number of distinct items fits in memory.
• Provide an approximate solution when distinct items are too many (state the error/accuracy tradeoff).

Explain time and space complexity for both approaches.