new_contents: class MinStack:
  
      def __init__(self):
          """
          initialize your data structure here.
          """
          self.data = []
          self.minVals = []
  
      def push(self, x):
          """
          :type x: int
          :rtype: void
          """
          self.data.append(x)
          if self.minVals:
              self.minVals.append(min(self.minVals[-1], x))
          else:
              self.minVals.append(x)
  
      def pop(self):
          """
          :rtype: void
          """
          if self.data:
              del self.minVals[-1]
              self.data
  
      def top(self):
          """
          :rtype: int
          """
          if self.data:
              return self.data[-1]
  
      def getMin(self):
          """
          :rtype: int
          """
          if self.data:
              return self.minVals[-1]
  
subject: add 155