Binary Tree Node Structure in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a binary tree node?

A binary tree node is a basic unit of a binary tree that contains data and references to two child nodes: left and right.

beginner

What are the main parts of a binary tree node?

A binary tree node has three main parts:
1. Data (value stored)
2. Left child reference
3. Right child reference

beginner

Show a simple Python class for a binary tree node.

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

beginner

Why do binary tree nodes have left and right references?

Because each node can have up to two children, left and right, to form the binary tree structure.

beginner

What does it mean if a binary tree node's left or right reference is None?

It means that the node does not have a child on that side (left or right). It is a leaf or partial node.

What does a binary tree node typically contain?

A Data only

B Data, next node, previous node

C Data, left child, right child

D Left child, right child only

A binary tree node contains data and references to left and right child nodes.

In a binary tree node, what does 'None' for left or right child mean?

A No child node on that side

B The node is the root

C The child node exists

D The node has two children

None means there is no child node on that side.

Which of these is NOT part of a binary tree node?

A Data

B Left child reference

C Right child reference

D Parent reference

A basic binary tree node does not usually have a parent reference.

How many children can a binary tree node have at most?

A 1

B 2

C 3

D Unlimited

A binary tree node can have up to two children: left and right.

What is the role of the left and right references in a binary tree node?

A To point to child nodes

B To store data

C To point to parent nodes

D To count children

Left and right references point to the node's child nodes.

Describe the structure of a binary tree node and its components.

Think about what each node stores and how it connects to others.

Explain why binary tree nodes have left and right references instead of just one.

Consider how a binary tree grows and branches.

Practice

1. What does a typical binary tree node contain in its structure?

easy

A. A value and a reference to the parent node only

B. Only a value and a reference to the next node

C. A value, a reference to the left child, and a reference to the right child

D. A value and a list of all children nodes

Solution

1. Step 1: Understand binary tree node components

   A binary tree node typically stores a value and has two child references: left and right.

2. Step 2: Compare options with definition

   A value, a reference to the left child, and a reference to the right child matches this structure exactly, while others describe different data structures.

3. Final Answer:

   A value, a reference to the left child, and a reference to the right child -> Option C

4. Quick Check:

   Binary tree node = value + left + right [OK]

Hint: Binary tree nodes always have left and right child pointers [OK]

Common Mistakes:

• Confusing binary tree node with linked list node
• Thinking nodes have parent pointers by default
• Assuming children are stored in a list

2. Which of the following is the correct Python syntax to define a binary tree node class with attributes val, left, and right?

easy

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

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

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

D. class Node:
def __init__(self, val, left, right):
self.val = val
self.left = left
self.right = right

Solution

1. Step 1: Check constructor parameters and attribute assignments

   class Node: def __init__(self, val): self.val = val self.left = None self.right = None correctly initializes val and sets left and right to None by default.

2. Step 2: Validate syntax correctness

   class Node: def __init__(self, val): val = self left = None right = None incorrectly assigns variables; class Node: def __init__(self): self.val = None self.left = None self.right = None misses val parameter; class Node: def __init__(self, val, left, right): self.val = val self.left = left self.right = right requires left and right on init which is less common.

3. Final Answer:

   class Node: def __init__(self, val): self.val = val self.left = None self.right = None -> Option B

4. Quick Check:

   Constructor sets val and initializes children to None [OK]

Hint: Init val, set left and right to None inside __init__ [OK]

Common Mistakes:

• Not using self to assign attributes
• Forgetting to initialize left and right
• Incorrect parameter order or missing parameters

3. What will be the output of the following code snippet?

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

root = Node(10)
root.left = Node(5)
root.right = Node(15)
print(root.left.val, root.right.val)

medium

A. 15 5

B. 10 5

C. None None

D. 5 15

Solution

1. Step 1: Trace node creation and assignments

   Root node has value 10; left child is assigned a node with value 5; right child assigned node with value 15.

2. Step 2: Print left and right child values

   Printing root.left.val and root.right.val outputs 5 and 15 respectively.

3. Final Answer:

   5 15 -> Option D

4. Quick Check:

   Left child val = 5, Right child val = 15 [OK]

Hint: Print left.val and right.val after assignment to get child values [OK]

Common Mistakes:

• Printing root.val instead of children
• Confusing left and right child values
• Assuming children are None without assignment

4. Identify the error in the following binary tree node class definition:

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

medium

A. Attributes are assigned incorrectly; should use self.val, self.left, self.right

B. Missing return statement in __init__ method

C. Class name should be lowercase

D. No error; code is correct

Solution

1. Step 1: Review attribute assignments in __init__

   Attributes must be assigned to self to belong to the instance, e.g., self.val = val.

2. Step 2: Identify incorrect assignments

   Code assigns val = self.val which reverses assignment and does not use self for left and right.

3. Final Answer:

   Attributes must be assigned using self, not local variables -> Option A

4. Quick Check:

   Use self.attr = value to set instance variables [OK]

Hint: Always assign attributes with self.attribute = value [OK]

Common Mistakes:

• Assigning variables without self prefix
• Reversing assignment order
• Expecting return in __init__

5. Given the following binary tree node class, how would you create a tree with root value 1, left child 2, and right child 3, then print the root and its children values?

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

hard

A. root = Node(1)
root.left = Node(2)
root.right = Node(3)
print(root.val, root.left.val, root.right.val)

B. root = Node(1, Node(2), Node(3))
print(root.val, root.left.val, root.right.val)

C. root = Node()
root.val = 1
root.left = 2
root.right = 3
print(root.val, root.left, root.right)

D. root = Node(1)
root.left = 2
root.right = 3
print(root.val, root.left, root.right)

Solution

1. Step 1: Create root node with value 1

   Use Node(1) to create root with val=1 and children None.

2. Step 2: Assign left and right children as Node instances

   Assign root.left = Node(2) and root.right = Node(3) to create child nodes properly.

3. Step 3: Print values of root and children

   Print root.val, root.left.val, root.right.val to get 1 2 3.

4. Final Answer:

   Use separate Node instances for children and print their val attributes -> Option A

5. Quick Check:

   Children must be Node objects, not integers [OK]

Hint: Assign children as Node objects, then print their val attributes [OK]

Common Mistakes:

• Assigning children as integers instead of Node objects
• Using constructor with extra parameters (not defined)
• Printing child nodes directly without .val