## Challenge Statement

• 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.
• This challenge corresponds to LeetCode #155.

### 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`.

### 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``````

## Solution

Below is my solution and some test cases. This solution has a constant time complexity O(1) and a constant space complexity O(1).