Why Trees Exist and What Linked Lists and Arrays Cannot Do in DSA Python - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a root node for a tree.

DSA Python

class Node:
    def __init__(self, value):
        self.value = value
        self.children = []

root = Node([1])

Drag options to blanks, or click blank then click option'

A0

B10

CNone

D'root'

Attempts:

3 left

💡 Hint

Common Mistakes

Using None as a value for the root node

Using a number instead of a descriptive string

The root node should have a value, here we use the string 'root' to identify it clearly.

2fill in blank

medium

Complete the code to add a child node to the root.

DSA Python

child = Node('child1')
root.children.[1](child)

Drag options to blanks, or click blank then click option'

Ainsert

Bappend

Cadd

Dextend

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert without index

Using add which is not a list method

Use append to add a single child node to the children list.

3fill in blank

hard

Fix the error in the code to print all children values of the root node.

DSA Python

for child in root.[1]:
    print(child.value)

Drag options to blanks, or click blank then click option'

Achilds

Bchild

Cchildren

Dnodes

Attempts:

3 left

💡 Hint

Common Mistakes

Using child which is a variable, not an attribute

Using childs which is not defined

The list of child nodes is stored in children, so we iterate over root.children.

4fill in blank

hard

Fill both blanks to check if a node has children and print their values.

DSA Python

if len(node.[1]) [2] 0:
    for c in node.children:
        print(c.value)

Drag options to blanks, or click blank then click option'

Achildren

B>

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using == which only checks for exactly zero children

Using < which checks for less than zero children

We check if the length of children is greater than zero to know if the node has children.

5fill in blank

hard

Fill all three blanks to create a dictionary of node values and their number of children if children exist.

DSA Python

result = [1]: len(node.[2]) for node in nodes if len(node.children) [3] 0

Drag options to blanks, or click blank then click option'

Anode.value

Bchildren

C>

Dnodes

Attempts:

3 left

💡 Hint

Common Mistakes

Using nodes instead of children

Using == instead of > in the condition

This dictionary comprehension maps each node's value to the count of its children, only if it has children.

Practice

1. Why do trees exist as a data structure when we already have arrays and linked lists?

easy

A. Because trees can represent hierarchical relationships which arrays and linked lists cannot

B. Because trees use less memory than arrays and linked lists

C. Because trees are easier to implement than arrays and linked lists

D. Because trees always provide faster access than arrays and linked lists

Solution

1. Step 1: Understand the limitation of arrays and linked lists

   Arrays and linked lists store data in a linear order, which makes it hard to represent hierarchical or branching relationships.

2. Step 2: Identify the unique feature of trees

   Trees allow nodes to have multiple children, representing parent-child relationships naturally, which arrays and linked lists cannot do efficiently.

3. Final Answer:

   Trees represent hierarchical relationships which arrays and linked lists cannot -> Option A

4. Quick Check:

   Hierarchy needs trees = A [OK]

Hint: Trees model hierarchy; arrays and lists are linear [OK]

Common Mistakes:

• Thinking trees always use less memory
• Believing trees are simpler to implement
• Assuming trees always have faster access

2. Which of the following is the correct way to define a simple tree node class in Python?

easy

A. class Node:
def __init__(self, value):
self.value = value
self.next = None

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

C. class Node:
def __init__(self, value):
self.value = value
self.children = []

D. class Node:
def __init__(self, value):
self.value = value
self.prev = None

Solution

1. Step 1: Identify the tree node structure

   A tree node typically stores a value and a list of children nodes to represent branches.

2. Step 2: Match the class definition

   class Node: def __init__(self, value): self.value = value self.children = [] defines a node with a value and a children list, which fits a general tree node structure.

3. Final Answer:

   Node with value and children list -> Option C

4. Quick Check:

   Tree nodes have children list = D [OK]

Hint: Tree nodes have children list, not next or prev pointers [OK]

Common Mistakes:

• Using 'next' pointer like in linked lists
• Using 'prev' pointer which is for doubly linked lists
• Adding unrelated pointers like 'next' in trees

3. What will be the output of this code that tries to print values in a simple tree node?

class Node:
    def __init__(self, value):
        self.value = value
        self.children = []

root = Node(1)
child1 = Node(2)
child2 = Node(3)
root.children.append(child1)
root.children.append(child2)

for child in root.children:
    print(child.value)

medium

A. Error: 'Node' object is not iterable

B. 1 2 3

C. 1

D. 2 3

Solution

1. Step 1: Understand the tree structure created

   Root node has value 1 and two children nodes with values 2 and 3.

2. Step 2: Trace the loop printing child values

   The loop iterates over root.children, printing each child's value: 2 then 3.

3. Final Answer:

   2 3 -> Option D

4. Quick Check:

   Children values printed = A [OK]

Hint: Loop prints children values, not root value [OK]

Common Mistakes:

• Printing root value inside loop
• Confusing children list with node itself
• Expecting root value to print in loop

4. Find the error in this code that tries to add a child to a tree node:

class Node:
    def __init__(self, value):
        self.value = value
        self.children = None

root = Node(1)
child = Node(2)
root.children.append(child)

medium

A. append() method does not exist for lists

B. children should be initialized as an empty list, not None

C. Node class should not have a children attribute

D. Cannot create child nodes before root node

Solution

1. Step 1: Check children initialization

   children is set to None, so it is not a list and cannot use append().

2. Step 2: Identify correct initialization

   children should be an empty list [] to allow appending child nodes.

3. Final Answer:

   children should be initialized as an empty list, not None -> Option B

4. Quick Check:

   children must be list for append() [OK]

Hint: Initialize children as [] to use append() [OK]

Common Mistakes:

• Setting children to None instead of list
• Trying to append to None type
• Ignoring initialization of children attribute

5. You want to represent a company's organizational chart where each employee can have multiple subordinates. Which data structure is best and why?

hard

A. Tree, because it naturally models hierarchical relationships

B. Array, because it stores data in order

C. Linked list, because it connects employees one after another

D. Stack, because it follows last-in-first-out order

Solution

1. Step 1: Understand the problem context

   An organizational chart shows who reports to whom, forming a hierarchy with multiple subordinates per employee.

2. Step 2: Match data structure to hierarchy

   Trees allow each node (employee) to have multiple children (subordinates), perfectly modeling this hierarchy.

3. Final Answer:

   Tree, because it naturally models hierarchical relationships -> Option A

4. Quick Check:

   Hierarchy needs trees = B [OK]

Hint: Hierarchy means use trees, not linear structures [OK]

Common Mistakes:

• Choosing arrays or lists for hierarchical data
• Confusing stack usage with hierarchy
• Ignoring multiple children per node