Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

We maintain two stacks: a main stack for the actual values and a "min stack" that tracks the minimum value at each level. When pushing a value, we push it to the main stack and push the new minimum (min of current value and current top of min stack) to the min stack.

Time complexity is O(1) for all operations (push, pop, top, getMin). Space complexity is O(n) to store the auxiliary min stack.