Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• getMin() -- Retrieve the minimum element in the stack.
  class MinStack {    //使用两个栈，一个正常使用，一个保存已经压栈中最小的数，每次push和pop注意两个栈同时操作    private Stack
         
           min=new Stack
          
           ();    private Stack
           
             stack=new Stack
            
             ();    public void push(int x) {        stack.push(x);        if(min.empty()||x