Min Stack

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.

class MinStack {
    
    Stack<Integer> element = new Stack<Integer>();
    Stack<Integer> minStack = new Stack<Integer>();
    public void push(int x) 
    {
        // element.add(x);
        // if (minStack.isEmpty() || x <= minStack.peek()) 
        // {
        //       minStack.add(x);
        // }
        if(minStack.isEmpty())
        {
            minStack.add(x);
            element.add(x);
        }
        else
        {
            int a = minStack.peek();
            minStack.add(Math.min(x,a));
            element.add(x);
        }
    }

    public void pop() 
    {
        if(element.isEmpty()) return;
        // if (element.peek().equals(minStack.peek())) 
        // {
        //      minStack.pop();
        //  }
        minStack.pop();
        element.pop();
    }

    public int top() 
    {
        return element.peek();
        
    }

    public int getMin() 
    {
        return minStack.peek();
        
    }
}

reference: http://algorithms.tutorialhorizon.com/track-the-maximum-element-in-a-stack/