Problem:
Binary trees are defined recursively. A binary tree _T_ is a structure defined on a finite set of nodes that either    * contains no nodes, or   * is composed of three disjoint sets of nodes:   \- a root node.   \- a binary tree called its left subtree.   \- a binary tree called its right subtree.    Your task is to write a program which perform tree walks (systematically traverse all nodes in a tree) based on the following algorithms:    1. Print the root, the left subtree and right subtree (preorder).   2. Print the left subtree, the root and right subtree (inorder).   3. Print the left subtree, right subtree and the root (postorder).  Here, the given binary tree consists of _n_ nodes and evey node has a unique ID from 0 to _n_ -1.

Incorrect submission (verdict: Wrong Answer):
class Tree():
    def __init__(self):
        self.nodes = {}
        self.nodes[-1] = None
    def add_node(self, id):
        if id not in self.nodes:
            self.nodes[id] = Node(id)        
    def add_child(self, parent_id, left_id, right_id):
        self.add_node(parent_id)
        self.add_node(left_id)
        self.add_node(right_id)
        self.nodes[parent_id].add_child(self.nodes[left_id], self.nodes[right_id])
    
class Node():
    def __init__(self, id):
        self.id = id
        self.parent = self.left = self.right = None
        self.depth = self.height = 0

    def add_child(self, left, right):
        self.left = left
        self.right = right
        
        self.update_height()
        
        for child in self.children():
            child.parent = self
            child.update_depth()
    def update_height(self):
        if self.degree():
            self.height = max([child.height + 1 for child in self.children()])
            if self.parent:
                self.parent.update_height()
            
    def update_depth(self):
        self.depth = self.parent.depth + 1
        for child in self.children():
            child.update_depth()
                
    def nodetype(self):
        if self.parent:
            if self.degree():
                return 'internal node'
            else:
                return 'leaf'
        else:
            return 'root'
    def degree(self):
        return len(self.children())
    
    def children(self):
        return [child for child in [self.left, self.right] if child]
    
    def sibling(self):
        if self.parent and self.parent.degree() == 2:
            if self.parent.left == self:
                return self.parent.right.id
            else:
                return self.parent.left.id
        else:
            return -1

    def __str__(self):
        return 'node {}: parent = {}, sibling = {}, degree = {}, depth = {}, height = {}, {}'.format(
            self.id,
            self.parent.id if self.parent else - 1,
            self.sibling(),
            self.degree(),
            self.depth,
            self.height,
            self.nodetype())
    def walk(self, order):
        orders = {'Preorder':[self, self.left, self.right], 'Inorder':[self.left, self, self.right], 'Postorder':[self.left, self.right, self]}
        
        for node in orders[order]:
            if node == self:
                yield node
            elif node:
                for childnode in node.walk(order):
                    yield childnode
                
tree = Tree()

n = int(input())
for i in range(n):
    id, left, right = map(int, input().split())
    tree.add_child(id, left, right)
orders = ['Preorder', 'Inorder', 'Postorder']

for order in orders:
    print(order)
    print(*[node.id for node in tree.nodes[0].walk(order)])

Fixed, accepted submission:
class Tree():
    def __init__(self):
        self.nodes = {}
        self.nodes[-1] = None
    def add_node(self, id):
        if id not in self.nodes:
            self.nodes[id] = Node(id)        
    def add_child(self, parent_id, left_id, right_id):
        self.add_node(parent_id)
        self.add_node(left_id)
        self.add_node(right_id)
        self.nodes[parent_id].add_child(self.nodes[left_id], self.nodes[right_id])
    def root(self):
        for node in self.nodes.values():
            if node and node.nodetype() == 'root':
                return node
    
class Node():
    def __init__(self, id):
        self.id = id
        self.parent = self.left = self.right = None
        self.depth = self.height = 0

    def add_child(self, left, right):
        self.left = left
        self.right = right
        
        self.update_height()
        
        for child in self.children():
            child.parent = self
            child.update_depth()
    def update_height(self):
        if self.degree():
            self.height = max([child.height + 1 for child in self.children()])
            if self.parent:
                self.parent.update_height()
            
    def update_depth(self):
        self.depth = self.parent.depth + 1
        for child in self.children():
            child.update_depth()
                
    def nodetype(self):
        if self.parent:
            if self.degree():
                return 'internal node'
            else:
                return 'leaf'
        else:
            return 'root'
    def degree(self):
        return len(self.children())
    
    def children(self):
        return [child for child in [self.left, self.right] if child]
    
    def sibling(self):
        if self.parent and self.parent.degree() == 2:
            if self.parent.left == self:
                return self.parent.right.id
            else:
                return self.parent.left.id
        else:
            return -1

    def __str__(self):
        return 'node {}: parent = {}, sibling = {}, degree = {}, depth = {}, height = {}, {}'.format(
            self.id,
            self.parent.id if self.parent else - 1,
            self.sibling(),
            self.degree(),
            self.depth,
            self.height,
            self.nodetype())
    def walk(self, order):
        orders = {'Preorder':[self, self.left, self.right], 'Inorder':[self.left, self, self.right], 'Postorder':[self.left, self.right, self]}
        
        for node in orders[order]:
            if node == self:
                yield node
            elif node:
                for childnode in node.walk(order):
                    yield childnode
                
tree = Tree()

n = int(input())
for i in range(n):
    id, left, right = map(int, input().split())
    tree.add_child(id, left, right)
orders = ['Preorder', 'Inorder', 'Postorder']

for order in orders:
    print(order)
    print('',*[node.id for node in tree.root().walk(order)])