Detect if a Linked List is Circular in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start traversing the linked list from the head node.

DSA Typescript

let current = [1];

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

Anull

Btail

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from tail instead of head.

Assigning null to current which stops traversal immediately.

We start checking from the head of the linked list to detect if it is circular.

2fill in blank

medium

Complete the condition to detect if the linked list is circular by checking if the next node points back to head.

DSA Typescript

if (current.next === [1]) {
  return true;
}

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

Anull

Bhead

Ccurrent

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if next is null instead of head.

Comparing with current instead of head.

If the next of the current node points back to head, the list is circular.

3fill in blank

hard

Fix the error in moving to the next node in the linked list traversal.

DSA Typescript

current = current[1];

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

Anext

Btail

Chead

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev which is not defined in singly linked list.

Assigning current to head or tail incorrectly.

To move forward in the linked list, assign current to current.next.

4fill in blank

hard

Fill both blanks to complete the while loop that traverses the linked list until it reaches the end.

DSA Typescript

while (current !== [1] && current.next !== [2]) {
  current = current.next;
}

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

Anull

Bhead

Cundefined

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Using head or tail instead of null in the condition.

Checking only one condition and causing runtime errors.

The loop continues while current is not null and current.next is not null, meaning the list has not ended.

5fill in blank

hard

Fill all three blanks to complete the function that returns false if no circular link is found after traversal.

DSA Typescript

function isCircular(head: Node | null): boolean {
  if (!head) return false;
  let current = head;
  while (current !== [1]) {
    if (current.next === [2]) {
      return true;
    }
    current = current.[3];
  }
  return false;
}

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

Anull

Bhead

Cnext

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined instead of null.

Not moving to the next node correctly.

Checking wrong conditions for circularity.

The function checks each node until current is null. If current.next points to head, it returns true. Otherwise, it moves to current.next. If no circular link is found, it returns false.

Practice

1. What does it mean if a linked list is circular?

easy

A. The last node points back to one of the previous nodes, forming a loop

B. The linked list has no nodes

C. The linked list nodes are sorted in ascending order

D. The linked list has only one node

Solution

1. Step 1: Understand linked list structure

   A linked list is a chain of nodes where each node points to the next node.

2. Step 2: Define circular linked list

   If the last node points back to any previous node, it creates a loop, making the list circular.

3. Final Answer:

   The last node points back to one of the previous nodes, forming a loop -> Option A

4. Quick Check:

   Circular linked list = last node points back [OK]

Hint: Circular means last node links back to earlier node [OK]

Common Mistakes:

• Thinking circular means sorted
• Confusing empty list with circular
• Assuming single node list is always circular

2. Which TypeScript code snippet correctly checks if a linked list is circular using two pointers?

easy

A. for(let node = head; node != null; node = node.next) { if(node.next === head) return true; } return false;

B. let slow = head; while(slow) { slow = slow.next; if(slow === head) return true; } return false;

C. let slow = head; let fast = head; while(fast && fast.next) { slow = slow.next; fast = fast.next.next; if(slow === fast) return true; } return false;

D. let current = head; while(current.next) { current = current.next; } return current === null;

Solution

1. Step 1: Understand Floyd's cycle detection

   Use two pointers: slow moves 1 step, fast moves 2 steps. If they meet, list is circular.

2. Step 2: Analyze code snippets

   let slow = head; let fast = head; while(fast && fast.next) { slow = slow.next; fast = fast.next.next; if(slow === fast) return true; } return false; correctly implements this logic with proper conditions and pointer moves.

3. Final Answer:

   let slow = head; let fast = head; while(fast && fast.next) { slow = slow.next; fast = fast.next.next; if(slow === fast) return true; } return false; -> Option C

4. Quick Check:

   Two pointers meet means circular [OK]

Hint: Use slow and fast pointers to detect loop [OK]

Common Mistakes:

• Moving fast pointer only one step
• Not checking fast and fast.next for null
• Returning false too early

3. What will be the output of this TypeScript code snippet?

class Node { constructor(public val: number, public next: Node | null = null) {} }
const head = new Node(1);
head.next = new Node(2);
head.next.next = new Node(3);
head.next.next.next = head.next;
function isCircular(head: Node | null): boolean {
  let slow = head;
  let fast = head;
  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;
    if (slow === fast) return true;
  }
  return false;
}
console.log(isCircular(head));

medium

A. true

B. false

C. undefined

D. Runtime error

Solution

1. Step 1: Understand the linked list structure

   Nodes 1 -> 2 -> 3, then 3 points back to node 2, creating a loop.

2. Step 2: Trace the two-pointer loop detection

   Slow and fast pointers will meet inside the loop, so function returns true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Loop exists, so output is true [OK]

Hint: Loop in list makes function return true [OK]

Common Mistakes:

• Assuming no loop because list looks linear
• Confusing node references
• Ignoring pointer equality check

4. Identify the bug in this TypeScript function meant to detect a circular linked list:

function isCircular(head: Node | null): boolean {
  let slow = head;
  let fast = head;
  while (fast !== null) {
    slow = slow.next;
    fast = fast.next.next;
    if (slow === fast) return true;
  }
  return false;
}

medium

A. The function should use a for loop instead of while

B. The slow pointer should move two steps instead of one

C. The function should return false inside the loop

D. The while loop should check fast and fast.next for null before moving pointers

Solution

1. Step 1: Analyze pointer movement safety

   fast.next.next is accessed without checking if fast.next is null, risking runtime error.

2. Step 2: Correct loop condition

   Loop should check both fast and fast.next are not null before moving pointers to avoid errors.

3. Final Answer:

   The while loop should check fast and fast.next for null before moving pointers -> Option D

4. Quick Check:

   Check fast and fast.next to avoid errors [OK]

Hint: Always check fast and fast.next before advancing fast [OK]

Common Mistakes:

• Not checking fast.next before fast.next.next
• Moving slow pointer incorrectly
• Returning false too early

5. Given a singly linked list where the last node might point to any node in the list or null, how would you modify the detection function to also return the node where the cycle begins if it is circular?
Which approach correctly finds the start of the loop?

hard

A. Count the number of nodes and return the middle node as the start of the loop

B. After detecting the cycle, reset one pointer to head and move both pointers one step at a time until they meet; the meeting point is the start of the loop

C. Use a hash set to store visited nodes and return the first repeated node found

D. Reverse the linked list and check if the head changes to detect the loop start

Solution

1. Step 1: Detect cycle using two pointers

   Use Floyd's algorithm to find if slow and fast pointers meet inside the loop.

2. Step 2: Find loop start node

   Reset one pointer to head, move both pointers one step at a time; their meeting point is the loop start.

3. Final Answer:

   After detecting the cycle, reset one pointer to head and move both pointers one step at a time until they meet; the meeting point is the start of the loop -> Option B

4. Quick Check:

   Reset pointer and move both to find loop start [OK]

Hint: Reset pointer after cycle detection to find loop start [OK]

Common Mistakes:

• Assuming middle node is loop start
• Using node count instead of pointers
• Trying to reverse list to find loop