Create and Initialize Doubly Linked List in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class for a doubly linked list node with a value property.

DSA Typescript

class Node {
  value: number;
  next: Node | null;
  prev: Node | null;

  constructor(value: number) {
    this.value = [1];
    this.next = null;
    this.prev = null;
  }
}

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

Anode

Bval

Cvalue

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name not declared in the constructor.

Assigning 'this.value' to 'val' or 'data' which are undefined.

The constructor parameter is named 'value', so to assign it to the property, use 'this.value = value;'.

2fill in blank

medium

Complete the code to initialize the head of the doubly linked list with a new node.

DSA Typescript

class DoublyLinkedList {
  head: Node | null;
  tail: Node | null;

  constructor(value: number) {
    const newNode = new Node(value);
    this.head = [1];
    this.tail = newNode;
  }
}

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

AnewNode.next

Bvalue

Cnull

DnewNode

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning head to null or to the value instead of the node.

Assigning head to newNode.next which is null.

The head should point to the new node created, so assign 'this.head = newNode;'.

3fill in blank

hard

Fix the error in the method that adds a new node at the end of the doubly linked list.

DSA Typescript

add(value: number) {
  const newNode = new Node(value);
  if (!this.head) {
    this.head = newNode;
    this.tail = newNode;
  } else {
    this.tail.next = [1];
    newNode.prev = this.tail;
    this.tail = newNode;
  }
}

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

Anull

BnewNode

Cthis.head

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning tail.next to value or null instead of the new node.

Assigning tail.next to head which breaks the list.

To link the current tail's next to the new node, assign 'this.tail.next = newNode;'.

4fill in blank

hard

Fill both blanks to complete the method that removes the last node from the doubly linked list.

DSA Typescript

remove() {
  if (!this.tail) return null;
  const temp = this.tail;
  if (this.head === this.tail) {
    this.head = [1];
    this.tail = [2];
  } else {
    this.tail = this.tail.prev;
    if (this.tail) this.tail.next = null;
    temp.prev = null;
  }
  return temp;
}

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

Anull

Bthis.head

Ctemp

Dthis.tail

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head or tail to the removed node instead of null.

Leaving head or tail pointing to a removed node.

When the list has one node, removing it sets both head and tail to null.

5fill in blank

hard

Fill all three blanks to complete the method that prints the doubly linked list values from head to tail.

DSA Typescript

printList() {
  let current = this.head;
  let result = '';
  while (current !== [1]) {
    result += current.value + ' <-> ';
    current = current.[2];
  }
  result += [3];
  console.log(result);
}

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

Anull

Bnext

C'null'

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'head' instead of null as loop end condition.

Using 'prev' instead of 'next' to move forward.

Not adding 'null' at the end of the printed string.

The loop runs until current is null, moving to current.next each time, and ends the string with 'null'.

Practice

1. What is the main difference between a doubly linked list and a singly linked list?

easy

A. A doubly linked list stores data in arrays instead of nodes.

B. A doubly linked list does not use pointers at all.

C. A doubly linked list can only be traversed forward.

D. A doubly linked list has nodes with pointers to both previous and next nodes.

Solution

1. Step 1: Understand node structure in doubly linked list

   Each node in a doubly linked list contains data and two pointers: one to the previous node and one to the next node.

2. Step 2: Compare with singly linked list

   A singly linked list node has only one pointer to the next node, so it can only be traversed forward.

3. Final Answer:

   A doubly linked list has nodes with pointers to both previous and next nodes. -> Option D

4. Quick Check:

   Doubly linked list = two pointers per node [OK]

Hint: Remember: doubly means two pointers per node [OK]

Common Mistakes:

• Thinking doubly linked list uses arrays
• Assuming it can only go forward
• Confusing pointer usage

2. Which of the following TypeScript code snippets correctly defines a node class for a doubly linked list?

easy

A. class Node { data: number; next: Node; constructor(data: number) { this.data = data; this.next = null; } }

B. class Node { data: number; next: Node | null; prev: Node | null; constructor(data: number) { this.data = data; this.next = null; this.prev = null; } }

C. class Node { data: number; prev: Node; constructor(data: number) { this.data = data; this.prev = null; } }

D. class Node { data: number; next: number; prev: number; constructor(data: number) { this.data = data; this.next = 0; this.prev = 0; } }

Solution

1. Step 1: Check for both next and prev pointers

   class Node { data: number; next: Node | null; prev: Node | null; constructor(data: number) { this.data = data; this.next = null; this.prev = null; } } defines both next and prev as Node or null, which is correct for doubly linked list nodes.

2. Step 2: Verify constructor initialization

   class Node { data: number; next: Node | null; prev: Node | null; constructor(data: number) { this.data = data; this.next = null; this.prev = null; } } initializes next and prev to null, which is proper for a new node.

3. Final Answer:

   class Node { data: number; next: Node | null; prev: Node | null; constructor(data: number) { this.data = data; this.next = null; this.prev = null; } } -> Option B

4. Quick Check:

   Node class needs next and prev pointers [OK]

Hint: Look for both next and prev properties initialized to null [OK]

Common Mistakes:

• Missing prev pointer in node class
• Using wrong types for pointers
• Initializing pointers with numbers instead of null

3. What will be the output of the following TypeScript code that creates a doubly linked list with nodes 10, 20, 30 and prints the list forward?

class Node {
  data: number;
  next: Node | null;
  prev: Node | null;
  constructor(data: number) {
    this.data = data;
    this.next = null;
    this.prev = null;
  }
}

class DoublyLinkedList {
  head: Node | null = null;

  append(data: number) {
    const newNode = new Node(data);
    if (!this.head) {
      this.head = newNode;
      return;
    }
    let current = this.head;
    while (current.next) {
      current = current.next;
    }
    current.next = newNode;
    newNode.prev = current;
  }

  printForward() {
    let current = this.head;
    let result = '';
    while (current) {
      result += current.data + ' -> ';
      current = current.next;
    }
    result += 'null';
    console.log(result);
  }
}

const list = new DoublyLinkedList();
list.append(10);
list.append(20);
list.append(30);
list.printForward();

medium

A. 10 -> 20 -> 30 -> null

B. 30 -> 20 -> 10 -> null

C. null -> 10 -> 20 -> 30

D. 10 -> 20 -> null

Solution

1. Step 1: Append nodes 10, 20, 30 in order

   Nodes are added at the end, so list order is 10, then 20, then 30.

2. Step 2: Print list forward

   printForward traverses from head to next nodes, printing data with arrows, ending with null.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   Appending adds to tail, printForward shows 10 to 30 [OK]

Hint: Appending adds nodes at tail; printForward shows head to tail [OK]

Common Mistakes:

• Confusing print order as reversed
• Missing last node in output
• Printing null at start instead of end

4. Identify the error in the following TypeScript code that tries to append a node to a doubly linked list:

class Node {
  data: number;
  next: Node | null;
  prev: Node | null;
  constructor(data: number) {
    this.data = data;
    this.next = null;
    this.prev = null;
  }
}

class DoublyLinkedList {
  head: Node | null = null;

  append(data: number) {
    const newNode = new Node(data);
    if (!this.head) {
      this.head = newNode;
      return;
    }
    let current = this.head;
    while (current.next) {
      current = current.next;
    }
    current.next = newNode;
    // Missing line here
  }
}

medium

A. The code misses setting newNode.prev to current before appending.

B. The code should set head to newNode even if head exists.

C. The constructor should initialize next and prev to undefined, not null.

D. The while loop should check current.prev instead of current.next.

Solution

1. Step 1: Check append logic for doubly linked list

   When appending, newNode.prev must point to current (the last node) to maintain backward link.

2. Step 2: Identify missing pointer assignment

   The code sets current.next = newNode but does not set newNode.prev = current, breaking backward link.

3. Final Answer:

   The code misses setting newNode.prev to current before appending. -> Option A

4. Quick Check:

   newNode.prev must link to previous node [OK]

Hint: Always set newNode.prev = last node when appending [OK]

Common Mistakes:

• Forgetting to set newNode.prev
• Changing head incorrectly
• Checking wrong pointer in loop

5. You want to create a doubly linked list from an array of numbers [5, 15, 25] and then reverse the list by changing pointers (not data). Which sequence of pointer changes correctly reverses the list?

hard

A. For each node, set next to prev and leave prev unchanged; head remains same.

B. For each node, set next to null and prev to null; then append nodes in reverse order.

C. For each node, swap its next and prev pointers; then update head to last node.

D. For each node, swap data values instead of pointers; head stays unchanged.

Solution

1. Step 1: Understand reversing a doubly linked list by pointers

   Reversing means for each node, next and prev pointers must be swapped to reverse direction.

2. Step 2: Update head pointer

   After swapping pointers, head must point to the original tail (last node) to reflect new start.

3. Final Answer:

   For each node, swap its next and prev pointers; then update head to last node. -> Option C

4. Quick Check:

   Swap next and prev, then reset head [OK]

Hint: Swap next and prev pointers on all nodes, then reset head [OK]

Common Mistakes:

• Only swapping data, not pointers
• Not updating head after reversal
• Setting pointers to null incorrectly