Traversal Forward and Backward in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does forward traversal mean in a linked list?

Forward traversal means moving from the first node to the last node, visiting each node one by one in order.

beginner

What is backward traversal in a doubly linked list?

Backward traversal means moving from the last node to the first node, visiting each node in reverse order using the previous links.

beginner

Why can't we do backward traversal in a singly linked list?

Because singly linked lists only have links to the next node, not to the previous one, so we cannot move backward easily.

beginner

In TypeScript, which property do you use to move forward in a doubly linked list node?

You use the next property to move forward to the next node.

beginner

In TypeScript, which property do you use to move backward in a doubly linked list node?

You use the prev property to move backward to the previous node.

Which traversal direction is NOT possible in a singly linked list without extra memory?

A Backward traversal

B Both forward and backward traversal

C Forward traversal

D None of the above

Singly linked lists only have links to the next node, so backward traversal is not possible without extra memory.

In a doubly linked list, which pointer helps in backward traversal?

A prev

B next

C head

D tail

The 'prev' pointer points to the previous node, enabling backward traversal.

What is the first step in forward traversal of a linked list?

A Start at the tail node

B Start at the head node

C Start at the middle node

D Start at any random node

Forward traversal always starts at the head (first) node.

Which data structure allows easy traversal both forward and backward?

A Queue

B Singly linked list

C Stack

D Doubly linked list

Doubly linked lists have pointers to both next and previous nodes, allowing traversal in both directions.

If you want to print all nodes from last to first, which traversal do you use?

A Forward traversal

B Depth-first traversal

C Backward traversal

D Random traversal

Backward traversal visits nodes from last to first.

Explain how forward and backward traversal work in a doubly linked list.

Think about which pointers connect nodes in each direction.

Why is backward traversal not straightforward in singly linked lists? How can it be achieved if needed?

Consider the links available in singly linked lists.

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