input: Write a function to implement a stack with a method to get the minimum value.
  assert stack.push(3) == None
  assert stack.push(5) == None
  assert stack.get_min() == 3
  assert stack.push(2) == None
  assert stack.get_min() == 2
  assert stack.pop() == 2
  assert stack.get_min() == 3
output: class MinStack:
      def __init__(self):
          self.stack = []
          self.min_stack = []
  
      def push(self, val: int) -> None:
          self.stack.append(val)
          if not self.min_stack or val <= self.min_stack[-1]:
              self.min_stack.append(val)
  
      def pop(self) -> int:
          if self.stack[-1] == self.min_stack[-1]:
              self.min_stack.pop()
          return self.stack.pop()
  
      def get_min(self) -> int:
          return self.min_stack[-1]
  
  stack = MinStack()