如何在O(1)内找到实时序列的最小值？

class MinStack(object):
    def __init__(self):

        """
        initialize your data structure here.
        """
        self.mainstack= []
        self.tmpstack = []

    def push(self, val):

        """
        :type val: int
        :rtype: None
        """

        self.mainstack.append(val)

        if not self.tmpstack:

            self.tmpstack.append(len(self.mainstack)-1)

        # smaller than top of tmpstack
        if self.mainstack[self.tmpstack[-1]] > val:

            self.tmpstack.append(len(self.mainstack)-1) 

    def pop(self):
        """
        :rtype: None
        """

        # min val of tmp stack equals top of mainstack
        if self.tmpstack and self.tmpstack[-1] == len(self.mainstack)-1:
            self.tmpstack.pop()

        return self.mainstack.pop()

   def top(self):
       """
       :rtype: int
       """

       if self.mainstack:
           return self.mainstack[-1]

    def getMin(self):
        """
        :rtype: int
        """

        if self.tmpstack:
            return self.mainstack[self.tmpstack[-1]]