Design an Expression Tree With Evaluate Function — Python

# Time:  O(n)
# Space: O(h)

import abc 
from abc import ABCMeta, abstractmethod 


class Node:
    __metaclass__ = ABCMeta
    # define your fields here
    @abstractmethod
    def evaluate(self):
        pass


import operator


class NodeIter(Node):
    ops = {'+':operator.add, '-':operator.sub, '*':operator.mul, '/':operator.div}
    
    def __init__(self, val):
        self.val = val
        self.left = None 
        self.right = None
    
    def evaluate(self):
        result = [0]
        stk = [(1, (self, result))]
        while stk:
            step, args = stk.pop()
            if step == 1:
                node, ret = args
                if node.val.isdigit():
                    ret[0] = int(node.val)
                    continue
                ret1, ret2 = [0], [0]
                stk.append((2, (node, ret1, ret2, ret)))
                stk.append((1, (node.right, ret2)))
                stk.append((1, (node.left, ret1)))
            elif step == 2:
                node, ret1, ret2, ret = args
                ret[0] = NodeIter.ops[node.val](ret1[0], ret2[0])
        return result[0]


class TreeBuilder(object):
    def buildTree(self, postfix):
        """
        :type s: List[str]
        :rtype: int
        """
        stk = []
        for c in postfix:
            if c.isdigit():
                stk.append(NodeIter(c))
            else:
                node = NodeIter(c)
                node.right = stk.pop()
                node.left = stk.pop()
                stk.append(node)                
        return stk.pop()


# Time:  O(n)
# Space: O(h)
class NodeRecu(Node):
    ops = {'+':operator.add, '-':operator.sub, '*':operator.mul, '/':operator.div}
    
    def __init__(self, val):
        self.val = val
        self.left = None 
        self.right = None

    def evaluate(self):
        if self.val.isdigit():
            return int(self.val)
        return NodeRecu.ops[self.val](self.left.evaluate(), self.right.evaluate())
        

class TreeBuilder2(object):
    def buildTree(self, postfix):
        """
        :type s: List[str]
        :rtype: int
        """
        stk = []
        for c in postfix:
            if c.isdigit():
                stk.append(NodeRecu(c))
            else:
                node = NodeRecu(c)
                node.right = stk.pop()
                node.left = stk.pop()
                stk.append(node)
        return stk.pop()