Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
Implement the `MinStack` class:
- `MinStack()` initializes the stack object.
- `void push(int val)` pushes the element val onto the stack.
- `void pop()` removes the element on the top of the stack.
- `int top()` gets the top element of the stack.
- `int getMin()` retrieves the minimum element in the stack.
checklist Constraints
-2^31 <= val <= 2^31 - 1 Methods pop, top and getMin operations will always be called on non-empty stacks. At most 3 * 10^4 calls will be made to push, pop, top, and getMin.