Leetcode 1628. Design an Expression Tree With Evaluate Function [Python]

System Design。使用stack存入数字，当遇到操作符号时，弹出stack中的数字。先出的为right，后出为left。完成操作后再压入stack。操作过程写在tree的evaluate函数里。

import abc
from abc import ABC, abstractmethod
"""
This is the interface for the expression tree Node.
You should not remove it, and you can define some classes to implement it.
"""class Node(ABC):@abstractmethod# define your fields heredef evaluate(self) -> int:passclass postfixtree:def __init__(self, val):self.val = valself.left = Noneself.right = Nonedef evaluate(self):if not self.left and not self.right:return self.valelif self.val == '+':return self.left.evaluate() + self.right.evaluate()elif self.val == '-':return self.left.evaluate() - self.right.evaluate()elif self.val == '*':return self.left.evaluate() * self.right.evaluate()elif self.val == '/':return self.left.evaluate() // self.right.evaluate()
"""
This is the TreeBuilder class.
You can treat it as the driver code that takes the postinfix input
and returns the expression tree represnting it as a Node.
"""class TreeBuilder(object):def buildTree(self, postfix: List[str]) -> 'Node':stack = []for index, char in enumerate(postfix):if char.isdigit():node = postfixtree(int(char))stack.append(node)else:right = stack.pop()left = stack.pop()node = postfixtree(char)node.left = leftnode.right = rightstack.append(node)return stack[-1]"""
Your TreeBuilder object will be instantiated and called as such:
obj = TreeBuilder();
expTree = obj.buildTree(postfix);
ans = expTree.evaluate();
"""

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