Create and Initialize Doubly Linked List in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows moving forward and backward through the list.

beginner

What are the main parts of a node in a doubly linked list?

Each node has: 1) data (the value stored), 2) next pointer (points to the next node), 3) previous pointer (points to the previous node).

beginner

How do you initialize an empty doubly linked list in TypeScript?

You create a class with a head and tail pointer set to null, meaning the list has no nodes yet.

intermediate

Why do we need both head and tail pointers in a doubly linked list?

Head points to the first node and tail points to the last node. This helps to quickly add or remove nodes at both ends and traverse the list in both directions.

beginner

Show the basic TypeScript class structure for a doubly linked list node.

class Node {
  data: any;
  next: Node | null;
  prev: Node | null;

  constructor(data: any) {
    this.data = data;
    this.next = null;
    this.prev = null;
  }
}

What does the 'prev' pointer in a doubly linked list node point to?

A The next node in the list

B The previous node in the list

C The head of the list

D The tail of the list

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

When initializing an empty doubly linked list, what are the values of head and tail?

A Head is null, tail points to a node

B Both point to a new node

C Both are null

D Head points to a node, tail is null

An empty list has no nodes, so both head and tail are null.

Which of these is NOT a part of a doubly linked list node?

A Random pointer

B Next pointer

C Previous pointer

D Data

A standard doubly linked list node has data, next, and previous pointers. Random pointer is not standard.

Why might you use a doubly linked list instead of a singly linked list?

A To save memory

B To make the list circular

C To avoid using pointers

D To allow traversal in both directions

Doubly linked lists allow moving forward and backward through the list.

In TypeScript, how do you declare a node's next pointer that can be null?

A next: Node | null;

B next: any;

C next: Node;

D next: undefined;

The next pointer can be a Node or null, so use a union type Node | null.

Explain how to create and initialize an empty doubly linked list in TypeScript.

Think about what pointers an empty list has and how nodes are structured.

Describe the role of the next and prev pointers in a doubly linked list node.

Consider how you move through the list in both directions.

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