155. Min Stack max stack Maxpop O（1） 操作

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.
Example:
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

Max Stack： 实现用ddl + treeset.  Node n = set.first() 

public class MinStack {
    Stack<Integer> stack;
    int min=Integer.MAX_VALUE;;
    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
    }
    
    public void push(int x) {
        if(x<=min)
        {
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }
    
    public void pop() {
        if(stack.pop() == min)
            min = stack.pop();
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int getMin() {
        return min;
    }
    
}