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.
You must implement a solution with O(1) time complexity for each function.
**Example:**
```
Input: ["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]
Output: [null,null,null,null,-3,null,0,-2]
```