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.

Example 1:

    Input
    ["MinStack","push","push","push","getMin","pop","top","getMin"]
    [[],[-2],[0],[-3],[],[],[],[]]

    Output
    [null,null,null,null,-3,null,0,-2]

    Explanation
    MinStack minStack = new MinStack();
    minStack.push(-2);
    minStack.push(0);
    minStack.push(-3);
    minStack.getMin(); // return -3
    minStack.pop();
    minStack.top();    // return 0
    minStack.getMin(); // return -2

Constraints:

- `-231 <= val <= 231 - 1`
- Methods `pop`, `top` and `getMin` operations will always be called on non-empty stacks.
- At most `3 * 104` calls will be made to `push`, `pop`, `top`, and `getMin`.