Adjacency List Representation in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize an empty adjacency list for a graph with 3 nodes.

DSA Python

graph = {i: [] for i in range([1])}

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

A3

B4

C5

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using range(4) creates an extra node.

Using range(2) misses one node.

The graph has 3 nodes numbered 0, 1, and 2. Using range(3) creates keys for these nodes with empty lists as values.

2fill in blank

medium

Complete the code to add an edge from node 0 to node 2 in the adjacency list.

DSA Python

graph[0].[1](2)

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

Aremove

Bextend

Cappend

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using extend with a single integer causes an error.

Using remove deletes elements instead of adding.

Use append to add node 2 to the adjacency list of node 0.

3fill in blank

hard

Fix the error in the code to correctly add an edge from node 1 to node 0.

DSA Python

graph[1].[1](0)

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

Aadd

Binsert

Cextend

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using add causes an AttributeError.

Using extend with an integer causes a TypeError.

append is the correct method to add a single neighbor to the adjacency list.

4fill in blank

hard

Fill both blanks to create an adjacency list for nodes 0 to 4 and add an edge from node 3 to node 1.

DSA Python

graph = {i: [] for i in range([1])}
graph[3].[2](1)

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

A5

Bappend

Cinsert

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using range(4) misses node 4.

Using insert adds at a specific position but is unnecessary here.

Use range(5) to create nodes 0 to 4. Use append to add node 1 to node 3's adjacency list.

5fill in blank

hard

Fill all three blanks to create an adjacency list for nodes 0 to 3, add an edge from node 2 to node 0, and from node 0 to node 3.

DSA Python

graph = {i: [] for i in range([1])}
graph[2].[2](0)
graph[0].[3](3)

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

A4

Bappend

Cinsert

Dextend

Attempts:

3 left

💡 Hint

Common Mistakes

Using extend with single integers causes errors.

Using insert is unnecessary here.

Use range(4) for nodes 0 to 3. Use append to add neighbors to adjacency lists.

Practice

1. What does an adjacency list represent in graph data structures?

easy

A. A list of all nodes without connections

B. A matrix showing distances between nodes

C. A list of nodes sorted by their values

D. A list where each node stores its connected neighbors

Solution

1. Step 1: Understand adjacency list concept

   An adjacency list stores each node with a list of nodes it connects to directly.

2. Step 2: Compare options with definition

   Only A list where each node stores its connected neighbors correctly describes this structure as neighbors stored per node.

3. Final Answer:

   A list where each node stores its connected neighbors -> Option D

4. Quick Check:

   Adjacency list = neighbors per node [OK]

Hint: Adjacency list = nodes with their direct neighbors [OK]

Common Mistakes:

• Confusing adjacency list with adjacency matrix
• Thinking it stores distances instead of neighbors
• Assuming it lists nodes without connections

2. Which of the following is the correct Python syntax to initialize an empty adjacency list for a graph with 3 nodes?

easy

A. graph = {0: [], 1: [], 2: []}

B. graph = [[], [], []]

C. graph = {0: None, 1: None, 2: None}

D. graph = [0, 1, 2]

Solution

1. Step 1: Understand adjacency list initialization

   An adjacency list is usually a dictionary with keys as nodes and values as lists of neighbors.

2. Step 2: Check each option

   graph = {0: [], 1: [], 2: []} correctly creates a dict with empty lists for each node. graph = [[], [], []] is a list of lists but lacks node keys. graph = {0: None, 1: None, 2: None} uses None instead of lists. graph = [0, 1, 2] is just a list of nodes, not adjacency lists.

3. Final Answer:

   graph = {0: [], 1: [], 2: []} -> Option A

4. Quick Check:

   Dict with empty lists = adjacency list init [OK]

Hint: Use dict with nodes as keys and empty lists as values [OK]

Common Mistakes:

• Using None instead of empty lists for neighbors
• Using list without node keys
• Confusing adjacency list with simple node list

3. What is the output of the following code?

graph = {0: [1, 2], 1: [2], 2: [0, 3], 3: []}
print(graph[2])

medium

A. [1, 2]

B. [0, 3]

C. [2]

D. []

Solution

1. Step 1: Identify the adjacency list for node 2

   In the dictionary, key 2 maps to the list [0, 3].

2. Step 2: Print the value for key 2

   Printing graph[2] outputs [0, 3].

3. Final Answer:

   [0, 3] -> Option B

4. Quick Check:

   graph[2] = [0, 3] [OK]

Hint: Look up the key in dict to get neighbors list [OK]

Common Mistakes:

• Confusing keys and values
• Printing wrong node's neighbors
• Misreading list contents

4. The following code tries to add an edge from node 1 to node 3 in the adjacency list but causes an error. What is the error?

graph = {0: [1], 1: [2], 2: [0]}
graph[1].append(3)
print(graph)

medium

A. KeyError because node 3 does not exist in graph

B. IndexError because list index out of range

C. No error, code runs correctly

D. TypeError because append is used on a dict

Solution

1. Step 1: Check if node 1 exists in graph

   Node 1 is a key in graph with value [2], so graph[1] is a list.

2. Step 2: Append 3 to graph[1]

   Appending 3 to the list [2] is valid and updates graph[1] to [2, 3].

3. Final Answer:

   No error, code runs correctly -> Option C

4. Quick Check:

   Appending to list in dict value works [OK]

Hint: Appending to existing list key is valid, no error [OK]

Common Mistakes:

• Thinking node 3 must exist as key before adding
• Confusing append on list vs dict
• Expecting error when adding new neighbor

5. Given a graph represented by the adjacency list:

graph = {0: [1, 2], 1: [2], 2: [0], 3: [2]}

Which node(s) have no outgoing edges?

hard

A. None, all have outgoing edges

B. Node 1

C. Node 2

D. Node 3

Solution

1. Step 1: Check neighbors list for each node

   Node 0 connects to [1, 2], node 1 to [2], node 2 to [0], node 3 to [2].

2. Step 2: Identify nodes with empty neighbor lists

   Node 3 has neighbors [2], so it has outgoing edges. No node has an empty list, so none have no outgoing edges.

3. Final Answer:

   None, all have outgoing edges -> Option A

4. Quick Check:

   All adjacency lists non-empty = no node without edges [OK]

Hint: Check for empty neighbor lists to find no outgoing edges [OK]

Common Mistakes:

• Assuming node 3 has no edges because it appears last
• Confusing incoming edges with outgoing edges
• Ignoring non-empty lists