Min Stack

Implement a `MinStack` class with `push(x)`, `pop()`, `top()` and `get_min()`, all in O(1). Scanning for the minimum on demand is O(n) and defeats the exercise. Keep a second stack of minima alongside the values. The follow-up is always duplicates: if you only push to the min-stack on a strict improvement, popping one copy of a repeated minimum discards it while an equal value is still in the stack.