Adjacency List Representation in DSA Typescript - Build from Scratch

Adjacency List Representation

📖 Scenario: Imagine you are organizing a small social network where each person can be friends with others. You want to store who is friends with whom in a way that is easy to look up and update.

🎯 Goal: You will build an adjacency list representation of a graph using a TypeScript object. Each key will be a person's name, and the value will be a list of their friends' names.

📋 What You'll Learn

Create an adjacency list object with exact keys and values

Add a new person with an empty friends list

Add friends to a person's list

Print the adjacency list to show the connections

💡 Why This Matters

🌍 Real World

Adjacency lists are used to represent networks like social connections, road maps, or computer networks efficiently.

💼 Career

Understanding adjacency lists is important for roles in software development, data analysis, and network engineering where graph data structures are common.

1

Create the initial adjacency list

Create a variable called adjacencyList as an object with these exact entries: 'Alice': ['Bob', 'Charlie'], 'Bob': ['Alice'], 'Charlie': ['Alice']

DSA Typescript

// Your code here

Hint

Use a TypeScript object with string keys and array of strings as values.

2

Add a new person with no friends

Add a new key 'David' to adjacencyList with an empty array as value

DSA Typescript

const adjacencyList: { [key: string]: string[] } = {
  'Alice': ['Bob', 'Charlie'],
  'Bob': ['Alice'],
  'Charlie': ['Alice']
};
// Your code here

Hint

Use bracket notation to add a new key with an empty array.

3

Add friends to David's list

Add 'Alice' and 'Bob' to adjacencyList['David'] array using push method

DSA Typescript

const adjacencyList: { [key: string]: string[] } = {
  'Alice': ['Bob', 'Charlie'],
  'Bob': ['Alice'],
  'Charlie': ['Alice']
};
adjacencyList['David'] = [];
// Your code here

Hint

Use push method twice to add two friends.

4

Print the adjacency list

Print the adjacencyList variable using console.log

DSA Typescript

const adjacencyList: { [key: string]: string[] } = {
  'Alice': ['Bob', 'Charlie'],
  'Bob': ['Alice'],
  'Charlie': ['Alice']
};
adjacencyList['David'] = [];
adjacencyList['David'].push('Alice');
adjacencyList['David'].push('Bob');
// Your code here

Hint

Use console.log(adjacencyList) to print the whole object.

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 queue of nodes to visit next

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 its neighbors, showing direct connections.

2. Step 2: Compare options with definition

   Only A list where each node stores its connected neighbors correctly describes this structure; others describe different concepts.

3. Final Answer:

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

4. Quick Check:

   Adjacency list = neighbors list [OK]

Hint: Adjacency list always stores neighbors per node [OK]

Common Mistakes:

• Confusing adjacency list with adjacency matrix
• Thinking it stores distances instead of neighbors
• Assuming it is a queue or unrelated structure

2. Which of the following is the correct TypeScript syntax to declare an adjacency list for a graph with nodes as numbers?

easy

A. const graph: number[][] = [];

B. const graph: Map = new Map();

C. const graph: Record = {};

D. const graph: Array<number> = [];

Solution

1. Step 1: Identify adjacency list type

   An adjacency list maps each node (number) to an array of connected nodes (number[]).

2. Step 2: Match TypeScript syntax

   Map correctly represents this; others are incorrect types or structures.

3. Final Answer:

   const graph: Map = new Map(); -> Option B

4. Quick Check:

   Map = adjacency list [OK]

Hint: Use Map with number keys and array values for adjacency list [OK]

Common Mistakes:

• Using Record which maps to single number, not array
• Using number[][] without keys for nodes
• Using Array<number> which is just a list, not adjacency

3. Given the following TypeScript code, what is the output after adding edges?

const graph: Map = new Map();
graph.set(1, [2, 3]);
graph.set(2, [1]);
graph.set(3, [1, 4]);
graph.set(4, [3]);
console.log(graph.get(3));

medium

A. [2, 3]

B. [3, 4]

C. [1, 4]

D. undefined

Solution

1. Step 1: Trace graph.set calls

   Node 3 is set with neighbors [1, 4].

2. Step 2: Check graph.get(3)

   Returns the array [1, 4] assigned to node 3.

3. Final Answer:

   [1, 4] -> Option C

4. Quick Check:

   graph.get(3) = [1, 4] [OK]

Hint: graph.get(node) returns neighbors array [OK]

Common Mistakes:

• Confusing neighbors of node 3 with node 1 or 2
• Expecting undefined if not careful with set calls
• Mixing up array contents order

4. Identify the error in this TypeScript adjacency list code snippet:

const graph: Map = new Map();
graph.set(1, [2]);
graph.set(2, [1]);
graph.get(3).push(1);

medium

A. graph.get(3) returns undefined, so push causes error

B. Map keys must be strings, not numbers

C. graph.set should use arrays of strings, not numbers

D. Missing initialization of graph variable

Solution

1. Step 1: Check graph.get(3) call

   Node 3 was never set, so graph.get(3) returns undefined.

2. Step 2: Understand push on undefined

   Calling push on undefined causes runtime error.

3. Final Answer:

   graph.get(3) returns undefined, so push causes error -> Option A

4. Quick Check:

   Accessing unset key returns undefined [OK]

Hint: Check if key exists before pushing to adjacency list [OK]

Common Mistakes:

• Assuming graph.get(3) returns empty array automatically
• Thinking Map keys must be strings (numbers are allowed)
• Ignoring runtime error from push on undefined

5. You want to represent a directed graph with nodes 1 to 4 using an adjacency list in TypeScript. Which code correctly adds an edge from node 2 to node 4 without affecting other edges?

const graph: Map = new Map();
graph.set(1, [2]);
graph.set(2, [3]);
graph.set(3, []);
graph.set(4, []);
// Add edge from 2 to 4 here

hard

A. graph.get(2)?.push(4);

B. graph.set(2, [4]);

C. graph.set(4, [2]);

D. graph.get(4)?.push(2);

Solution

1. Step 1: Understand existing edges

   Node 2 currently has neighbors [3]. We want to add 4 without removing 3.

2. Step 2: Choose correct method to add edge

   Using graph.get(2)?.push(4) adds 4 to existing array, preserving [3]. graph.set(2, [4]) replaces neighbors, removing 3.

3. Final Answer:

   graph.get(2)?.push(4); -> Option A

4. Quick Check:

   Use push to add edge without overwriting [OK]

Hint: Use push on existing neighbors to add edges safely [OK]

Common Mistakes:

• Overwriting neighbors array with set instead of pushing
• Adding edge to wrong node
• Confusing direction of edge in directed graph