155\. Min Stack

Easy

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:**

*   <code>-2<sup>31</sup> <= val <= 2<sup>31</sup> - 1</code>
*   Methods `pop`, `top` and `getMin` operations will always be called on **non-empty** stacks.
*   At most <code>3 * 10<sup>4</sup></code> calls will be made to `push`, `pop`, `top`, and `getMin`.