#Data Structures

Solve a medium level DSA problem that requires a solid understanding of algorithms and data structures. Describe your approach in detail.

Write a function to reverse a singly linked list. The function should take the head of the list as input and return the new head after reversal. For example, given the list 1 -> 2 -> 3 -> NULL, it should return 3 -> 2 -> 1 -> NULL.

Implement a Min Stack that includes the ability to retrieve the minimum element in O(1) time. The stack should support standard stack operations: push, pop, and top.

You will engage in a discussion about algorithms and data structures. Be prepared to ask clarifying questions, start with a brute-force approach and then optimize, and explain your understanding of time and space complexity clearly.

Can you explain how a hashmap works internally?