Clone Linked List with Random Pointer in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a 'random pointer' in a linked list?

A random pointer is an extra pointer in each node that can point to any node in the list or null, not just the next node.

intermediate

Why do we need to clone the random pointer separately when copying a linked list?

Because the random pointer can point anywhere in the original list, we must carefully assign the corresponding random pointer in the cloned list to maintain the same structure.

intermediate

What are the three main steps to clone a linked list with random pointers?

1. Insert cloned nodes between original nodes.
2. Assign random pointers for cloned nodes.
3. Separate the cloned list from the original list.

intermediate

In the cloning process, how do we assign the random pointer of the cloned node?

For each original node, the cloned node's random pointer is set to the next node of the original node's random pointer (if it exists).

beginner

What is the time complexity of cloning a linked list with random pointers using the three-step method?

The time complexity is O(n), where n is the number of nodes, because we traverse the list a constant number of times.

What does the random pointer in a linked list node point to?

A Any node in the list or null

B Only the next node

C Only the previous node

D The head of the list

The random pointer can point to any node in the list or be null.

What is the first step in cloning a linked list with random pointers?

A Assign random pointers to cloned nodes

B Separate cloned list from original

C Insert cloned nodes between original nodes

D Reverse the original list

First, cloned nodes are inserted between original nodes to keep track of their positions.

How do you set the random pointer of a cloned node?

A Clone node's random = original node's random

B Clone node's random = null

C Clone node's random = original node's next node

D Clone node's random = original node's random's next node

The cloned node's random pointer is set to the next node of the original node's random pointer.

What is the time complexity of cloning a linked list with random pointers using the three-step method?

A O(n^2)

B O(n)

C O(log n)

D O(1)

The cloning process takes linear time, O(n), as it traverses the list a few times.

After cloning, how do you separate the cloned list from the original list?

A By skipping every other node starting from head

B By reversing the list

C By deleting original nodes

D By copying values only

You separate the cloned list by skipping every other node, extracting the cloned nodes.

Explain the three main steps to clone a linked list with random pointers.

Think about how to keep track of cloned nodes and their random pointers.

Describe how to assign the random pointer for each cloned node during the cloning process.

Remember cloned nodes are inserted right after original nodes.

Practice

1. What is the main challenge when cloning a linked list with a random pointer?

easy

A. Reversing the linked list before cloning

B. Sorting the linked list before cloning

C. Copying the next pointers correctly while preserving the random pointers

D. Ignoring the random pointers during cloning

Solution

1. Step 1: Understand the structure of the linked list

   The linked list has two pointers: next and random. The next pointer points to the next node, and the random pointer can point to any node or null.

2. Step 2: Identify the cloning challenge

   While cloning, copying next pointers is straightforward, but random pointers can point anywhere, so they must be copied carefully to preserve the original structure.

3. Final Answer:

   Copying the next pointers correctly while preserving the random pointers -> Option C

4. Quick Check:

   Random pointer preservation = main cloning challenge [OK]

Hint: Remember random pointers can point anywhere, handle them carefully [OK]

Common Mistakes:

• Ignoring random pointers during cloning
• Assuming random pointers follow next pointers
• Trying to sort or reverse the list before cloning

2. Which of the following C# code snippets correctly declares a node class for a linked list with a random pointer?

easy

A. public class Node { public int val; public Node next; public Node random; public Node(int val) { this.val = val; next = null; random = null; } }

B. public class Node { int val; Node next; Node random; }

C. class Node { public int val; public Node next; }

D. public struct Node { public int val; public Node next; public Node random; }

Solution

1. Step 1: Check for correct access modifiers and constructor

   public class Node { public int val; public Node next; public Node random; public Node(int val) { this.val = val; next = null; random = null; } } declares a public class with public fields and a constructor initializing val, next, and random to null, which is correct.

2. Step 2: Identify issues in other options

   public class Node { int val; Node next; Node random; } lacks access modifiers and constructor. class Node { public int val; public Node next; } misses random pointer. public struct Node { public int val; public Node next; public Node random; } uses struct which is not ideal for linked list nodes.

3. Final Answer:

   Option A code snippet correctly declares the node class -> Option A

4. Quick Check:

   Public class with constructor = correct node declaration [OK]

Hint: Use public class with constructor initializing all pointers [OK]

Common Mistakes:

• Missing public access modifiers
• Not initializing pointers in constructor
• Using struct instead of class for nodes

3. Given the following code snippet for cloning a linked list with random pointers, what will be the output of the cloned list's node values in order?

Node head = new Node(1);
head.next = new Node(2);
head.next.next = new Node(3);
head.random = head.next.next; // 1's random points to 3
head.next.random = head; // 2's random points to 1
head.next.next.random = head.next; // 3's random points to 2

Node clonedHead = CloneLinkedList(head);
Node current = clonedHead;
while (current != null) {
    Console.Write(current.val + " ");
    current = current.next;
}

medium

A. 3 2 1

B. 1 2 3

C. 1 3 2

D. 2 1 3

Solution

1. Step 1: Understand the original list structure

   The original list nodes are 1 -> 2 -> 3 with next pointers in that order. Random pointers do not affect the order of next pointers.

2. Step 2: Cloning preserves next order

   The CloneLinkedList function clones nodes preserving the next pointer order, so the cloned list will have nodes 1 -> 2 -> 3 in sequence.

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   Next pointers order = 1 2 3 [OK]

Hint: Cloning preserves next order; random pointers don't change sequence [OK]

Common Mistakes:

• Confusing random pointer order with next pointer order
• Assuming cloned list reverses nodes
• Printing random pointer values instead of node values

4. Identify the bug in the following CloneLinkedList method that clones a linked list with random pointers:

public Node CloneLinkedList(Node head) {
    if (head == null) return null;
    Node current = head;
    while (current != null) {
        Node copy = new Node(current.val);
        copy.next = current.next;
        current.next = copy;
        current = current.next;
    }
    // Setting random pointers omitted for brevity
    return head.next;
}

medium

A. The method returns the original list head instead of cloned head

B. Random pointers are not set, so cloned list is incomplete

C. The copy node's next pointer is set incorrectly

D. The current pointer update causes an infinite loop

Solution

1. Step 1: Analyze the while loop pointer update

   Inside the loop, current is updated with current = current.next, but after insertion, current.next points to the copy node, so current moves to copy node again, causing an infinite loop.

2. Step 2: Correct pointer update to skip copy node

   To avoid infinite loop, current should be updated to current.next.next to move to the next original node.

3. Final Answer:

   The current pointer update causes an infinite loop -> Option D

4. Quick Check:

   Pointer update must skip copy node to avoid infinite loop [OK]

Hint: Update current to current.next.next after inserting copy [OK]

Common Mistakes:

• Updating current to current.next causes infinite loop
• Forgetting to set random pointers
• Returning original head instead of cloned head

5. You want to clone a linked list with random pointers in O(n) time and O(1) extra space. Which approach correctly achieves this?

hard

A. Interleave cloned nodes with original nodes, set random pointers, then separate lists

B. Reverse the original list, clone nodes, then reverse back

C. Create a new list by copying values only, ignoring random pointers

D. Use a hash map to store original to cloned node mappings

Solution

1. Step 1: Understand space and time constraints

   O(n) time and O(1) extra space means no extra data structures like hash maps can be used.

2. Step 2: Use interleaving method

   Interleave cloned nodes between original nodes, set random pointers using original nodes, then separate the two lists. This uses no extra space and runs in linear time.

3. Final Answer:

   Interleave cloned nodes with original nodes, set random pointers, then separate lists -> Option A

4. Quick Check:

   Interleaving nodes = O(n) time & O(1) space cloning [OK]

Hint: Interleave clones between originals to avoid extra space [OK]

Common Mistakes:

• Using hash map increases space to O(n)
• Ignoring random pointers during cloning
• Reversing list does not help cloning random pointers