Hi, here's your problem today. This problem was recently asked by Uber:

Design a simple 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.

Note: be sure that pop() and top() handle being called on an empty stack.

<pre lang=py>
class minStack(object):
  def __init__(self):
    # Fill this in.

  def push(self, x):
    # Fill this in.

  def pop(self):
    # Fill this in.

  def top(self):
    # Fill this in.

  def getMin(self):
    # Fill this in.

x = minStack()
x.push(-2)
x.push(0)
x.push(-3)
print(x.getMin())
# -3
x.pop()
print(x.top())
# 0
print(x.getMin())
# -2
</pre>