Traversal Forward and Backward in DSA Typescript - Build from Scratch

Traversal Forward and Backward

📖 Scenario: You are managing a playlist of songs. You want to move forward and backward through the songs to see which one is playing next or previous.

🎯 Goal: Build a simple doubly linked list of songs and practice moving forward and backward through the list.

📋 What You'll Learn

Create a doubly linked list with exactly three songs: 'SongA', 'SongB', and 'SongC'.

Create a variable to start at the head of the list.

Move forward to the next song and then backward to the previous song.

Print the current song's name after moving forward and backward.

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, browsers, and other apps where you need to move forward and backward through items.

💼 Career

Understanding traversal in doubly linked lists helps in software development roles that involve data structure manipulation and UI navigation.

1

Create the doubly linked list with three songs

Create a class called Node with properties song (string), next (Node or null), and prev (Node or null). Then create three nodes with songs 'SongA', 'SongB', and 'SongC'. Link them so that SongA points to SongB as next, SongB points to SongC as next, and the prev pointers go backward accordingly. Finally, create a variable called head that points to the first node (SongA).

DSA Typescript

# Your code here

Hint

Think of each song as a box that knows the next and previous boxes.

2

Create a variable to start at the head

Create a variable called current and set it equal to head to start at the first song.

DSA Typescript

class Node {
    song: string;
    next: Node | null;
    prev: Node | null;

    constructor(song: string) {
        this.song = song;
        this.next = null;
        this.prev = null;
    }
}

const node1 = new Node('SongA');
const node2 = new Node('SongB');
const node3 = new Node('SongC');

node1.next = node2;
node2.prev = node1;
node2.next = node3;
node3.prev = node2;

const head = node1;

# Create a variable called current and set it to head
# Your code here

Hint

Use const current = head; to start at the first song.

3

Move forward and then backward through the list

Move current forward to the next song by setting current to current.next. Then move current backward to the previous song by setting current to current.prev.

DSA Typescript

class Node {
    song: string;
    next: Node | null;
    prev: Node | null;

    constructor(song: string) {
        this.song = song;
        this.next = null;
        this.prev = null;
    }
}

const node1 = new Node('SongA');
const node2 = new Node('SongB');
const node3 = new Node('SongC');

node1.next = node2;
node2.prev = node1;
node2.next = node3;
node3.prev = node2;

const head = node1;

const current = head;

# Move current forward to next song
# Move current backward to previous song
# Your code here

Hint

Use current = current.next! to move forward and current = current.prev! to move backward.

4

Print the current song after moving forward and backward

Print the song name of current using console.log(current.song).

DSA Typescript

class Node {
    song: string;
    next: Node | null;
    prev: Node | null;

    constructor(song: string) {
        this.song = song;
        this.next = null;
        this.prev = null;
    }
}

const node1 = new Node('SongA');
const node2 = new Node('SongB');
const node3 = new Node('SongC');

node1.next = node2;
node2.prev = node1;
node2.next = node3;
node3.prev = node2;

const head = node1;

let current = head;
current = current.next!;
current = current.prev!;

# Print the current song name
# Your code here

Hint

Use console.log(current.song) to print the current song.

Practice

1. What does forward traversal in a linked list mean?

easy

A. Visiting nodes randomly without order

B. Visiting nodes from the tail to the head in order

C. Skipping every other node while traversing

D. Visiting nodes from the head to the tail in order

Solution

1. Step 1: Understand linked list structure

   A linked list has nodes connected from head to tail in one direction.

2. Step 2: Define forward traversal

   Forward traversal means visiting nodes starting at the head and moving to the tail in order.

3. Final Answer:

   Visiting nodes from the head to the tail in order -> Option D

4. Quick Check:

   Forward traversal = head to tail [OK]

Hint: Forward means start at head and go next nodes [OK]

Common Mistakes:

• Confusing forward with backward traversal
• Thinking traversal is random
• Assuming skipping nodes is traversal

2. Which TypeScript syntax correctly traverses an array forward using a for loop?

easy

A. for(let i = arr.length; i >= 0; i--) { console.log(arr[i]); }

B. for(let i = 0; i < arr.length; i++) { console.log(arr[i]); }

C. for(let i = 1; i <= arr.length; i++) { console.log(arr[i]); }

D. for(let i = arr.length - 1; i > 0; i--) { console.log(arr[i]); }

Solution

1. Step 1: Identify forward traversal loop

   Forward traversal starts at index 0 and goes up to length-1.

2. Step 2: Check loop conditions

   for(let i = 0; i < arr.length; i++) { console.log(arr[i]); } loops from 0 to less than arr.length, printing each element correctly.

3. Final Answer:

   for(let i = 0; i < arr.length; i++) { console.log(arr[i]); } -> Option B

4. Quick Check:

   Forward loop index from 0 to length-1 [OK]

Hint: Start index 0, end before length for forward loop [OK]

Common Mistakes:

• Starting index at 1 instead of 0
• Using i <= arr.length causing out-of-bounds
• Looping backward for forward traversal

3. What is the output of this TypeScript code?

const arr = [10, 20, 30];
for(let i = arr.length - 1; i >= 0; i--) {
  console.log(arr[i]);
}

medium

A. 30 20 10

B. 10 20 30

C. 30 20

D. Error: Index out of bounds

Solution

1. Step 1: Understand loop start and end

   The loop starts at index 2 (arr.length - 1) and goes down to 0.

2. Step 2: Trace output values

   It prints arr[2] = 30, arr[1] = 20, arr[0] = 10 in that order.

3. Final Answer:

   30 20 10 -> Option A

4. Quick Check:

   Backward traversal prints 30 20 10 [OK]

Hint: Backward loop starts at length-1 down to 0 [OK]

Common Mistakes:

• Thinking output is forward order
• Missing last element at index 0
• Assuming error due to loop

4. Identify the error in this backward traversal code for a linked list:

let current = head;
while(current !== null) {
  console.log(current.value);
  current = current.prev;
}

medium

A. It should use current.next instead of current.prev

B. No error, code is correct

C. It should start from tail, not head

D. It should check current.next !== null in while condition

Solution

1. Step 1: Understand backward traversal in linked list

   Backward traversal uses prev pointers starting from the tail node.

2. Step 2: Check starting node

   The code starts at head, but to go backward, it must start at tail.

3. Final Answer:

   It should start from tail, not head -> Option C

4. Quick Check:

   Backward traversal starts at tail [OK]

Hint: Backward traversal must start at tail node [OK]

Common Mistakes:

• Starting backward traversal at head
• Using next instead of prev for backward
• Wrong loop condition check

5. Given a doubly linked list, how would you print all values backward starting from the head node?

hard

A. Traverse forward to tail, then traverse backward from tail to head

B. Traverse backward directly from head using prev pointers

C. Use only next pointers starting from head to print backward

D. Reverse the list first, then traverse forward

Solution

1. Step 1: Understand doubly linked list structure

   Each node has next and prev pointers; head has no prev, tail has no next.

2. Step 2: Plan backward print from head

   To print backward from head, first move forward to tail using next pointers.

3. Step 3: Traverse backward from tail

   From tail, use prev pointers to print nodes backward to head.

4. Final Answer:

   Traverse forward to tail, then traverse backward from tail to head -> Option A

5. Quick Check:

   Backward print from head requires forward then backward traversal [OK]

Hint: Go forward to tail, then backward to print [OK]

Common Mistakes:

• Trying to go backward directly from head
• Using next pointers to print backward
• Reversing list unnecessarily