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 getMi- n.