Min Stack

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 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`.