Design an Expression Tree With Evaluate Function solution and explanation

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 here
    def evaluate(self) -> int:
        pass


class MyNode(Node):
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

    def evaluate(self) -> int:
        x = self.val
        if x.isdigit():
            return int(x)

        left, right = self.left.evaluate(), self.right.evaluate()
        if x == '+':
            return left + right
        if x == '-':
            return left - right
        if x == '*':
            return left * right
        if x == '/':
            return left // right


"""
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':
        stk = []
        for s in postfix:
            node = MyNode(s)
            if not s.isdigit():
                node.right = stk.pop()
                node.left = stk.pop()
            stk.append(node)
        return stk[-1]


"""
Your TreeBuilder object will be instantiated and called as such:
obj = TreeBuilder();
expTree = obj.buildTree(postfix);
ans = expTree.evaluate();
"""

/**
 * This is the interface for the expression tree Node.
 * You should not remove it, and you can define some classes to implement it.
 */

abstract class Node {
    public abstract int evaluate();
    // define your fields here
    protected String val;
    protected Node left;
    protected Node right;
};

class MyNode extends Node {
    public MyNode(String val) {
        this.val = val;
    }

    public int evaluate() {
        if (isNumeric()) {
            return Integer.parseInt(val);
        }
        int leftVal = left.evaluate();
        int rightVal = right.evaluate();
        if (Objects.equals(val, "+")) {
            return leftVal + rightVal;
        }
        if (Objects.equals(val, "-")) {
            return leftVal - rightVal;
        }
        if (Objects.equals(val, "*")) {
            return leftVal * rightVal;
        }
        if (Objects.equals(val, "/")) {
            return leftVal / rightVal;
        }
        return 0;
    }

    public boolean isNumeric() {
        for (char c : val.toCharArray()) {
            if (!Character.isDigit(c)) {
                return false;
            }
        }
        return true;
    }
}

/**
 * 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 {
    Node buildTree(String[] postfix) {
        Deque<MyNode> stk = new ArrayDeque<>();
        for (String s : postfix) {
            MyNode node = new MyNode(s);
            if (!node.isNumeric()) {
                node.right = stk.pop();
                node.left = stk.pop();
            }
            stk.push(node);
        }
        return stk.peek();
    }
};

/**
 * Your TreeBuilder object will be instantiated and called as such:
 * TreeBuilder obj = new TreeBuilder();
 * Node expTree = obj.buildTree(postfix);
 * int ans = expTree.evaluate();
 */

/**
 * 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 {
public:
    virtual ~Node(){};
    virtual int evaluate() const = 0;

protected:
    // define your fields here
    string val;
    Node* left;
    Node* right;
};

class MyNode : public Node {
public:
    MyNode(string val) {
        this->val = val;
    }

    MyNode(string val, Node* left, Node* right) {
        this->val = val;
        this->left = left;
        this->right = right;
    }

    int evaluate() const {
        if (!(val == "+" || val == "-" || val == "*" || val == "/")) return stoi(val);
        auto leftVal = left->evaluate(), rightVal = right->evaluate();
        if (val == "+") return leftVal + rightVal;
        if (val == "-") return leftVal - rightVal;
        if (val == "*") return leftVal * rightVal;
        if (val == "/") return leftVal / rightVal;
        return 0;
    }
};

/**
 * 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 {
public:
    Node* buildTree(vector<string>& postfix) {
        stack<MyNode*> stk;
        for (auto s : postfix) {
            MyNode* node;
            if (s == "+" || s == "-" || s == "*" || s == "/") {
                auto right = stk.top();
                stk.pop();
                auto left = stk.top();
                stk.pop();
                node = new MyNode(s, left, right);
            } else {
                node = new MyNode(s);
            }
            stk.push(node);
        }
        return stk.top();
    }
};

/**
 * Your TreeBuilder object will be instantiated and called as such:
 * TreeBuilder* obj = new TreeBuilder();
 * Node* expTree = obj->buildTree(postfix);
 * int ans = expTree->evaluate();
 */