MinStack (leetcode-in-java 1.21 API)

java.lang.Object
- g0101_0200.s0155_min_stack.MinStack

```
public class MinStack
extends Object
```
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:
- -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.

- Constructor Summary
  
  Constructors
  Constructor Description
  
  MinStack()
- Method Summary
  
  All Methods Instance Methods Concrete Methods
  Modifier and Type Method Description
  
  int getMin()
  
  void pop()
  
  void push(int val)
  
  int top()
  - Methods inherited from class java.lang.Object
    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail
- MinStack
```
public MinStack()
```

Method Detail

push
```
public void push(int val)
```

pop
```
public void pop()
```

top
```
public int top()
```

getMin
```
public int getMin()
```