Detect Cycle in Linked List Floyd's Algorithm in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the slow pointer at the head of the linked list.

DSA Javascript

let slow = [1];

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

Ahead

Bnull

Cfast

Dslow

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing slow to null will cause the loop to never start.

Using fast instead of head confuses the two pointers.

The slow pointer should start at the head of the linked list to begin traversal.

2fill in blank

medium

Complete the condition to continue the loop while both pointers can move forward.

DSA Javascript

while (fast !== null && fast.next !== [1]) {

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

Ahead

Bnull

Cslow

Dfast

Attempts:

3 left

💡 Hint

Common Mistakes

Using head or slow in the condition causes incorrect checks.

Not checking fast.next leads to runtime errors.

The loop continues only if fast and fast.next are not null to avoid errors.

3fill in blank

hard

Fix the error in moving the fast pointer two steps ahead.

DSA Javascript

fast = fast.[1].next;

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

Anext

Bprev

Cchild

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' or 'child' properties which do not exist in singly linked lists.

Moving only one step instead of two.

The fast pointer moves two steps ahead by accessing fast.next.next.

4fill in blank

hard

Fill both blanks to update slow and fast pointers correctly inside the loop.

DSA Javascript

slow = slow.[1];
fast = fast.[2].next;

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

Anext

Bprev

Cchild

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' or 'child' which are not part of singly linked lists.

Moving fast only one step instead of two.

Both slow and fast pointers move forward by accessing the 'next' property of nodes.

5fill in blank

hard

Fill all three blanks to complete the cycle detection check and return statement.

DSA Javascript

if (slow === [1]) {
  return [2];
}
return [3];

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

Afast

Btrue

Cfalse

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing slow to head instead of fast.

Returning wrong boolean values.

If slow and fast meet, a cycle exists, so return true; otherwise, return false.

Practice

1. What is the main purpose of Floyd's Cycle Detection Algorithm in a linked list?

easy

A. To reverse the linked list

B. To sort the linked list in ascending order

C. To find the middle element of the linked list

D. To check if the linked list contains a cycle or loop

Solution

1. Step 1: Understand Floyd's Algorithm goal

   Floyd's Cycle Detection Algorithm is designed to detect if a linked list has a cycle, meaning a node points back to a previous node.

2. Step 2: Compare with other linked list operations

   Sorting, reversing, or finding the middle element are different operations and not the purpose of this algorithm.

3. Final Answer:

   To check if the linked list contains a cycle or loop -> Option D

4. Quick Check:

   Detect cycle = Check loop presence [OK]

Hint: Floyd's algorithm detects loops, not sorting or reversing [OK]

Common Mistakes:

• Confusing cycle detection with sorting
• Thinking it finds the middle node
• Assuming it reverses the list

2. Which of the following is the correct way to initialize the two pointers in Floyd's Cycle Detection Algorithm in JavaScript?

easy

A. let slow = null; let fast = head;

B. let slow = head; let fast = head;

C. let slow = head; let fast = head.next;

D. let slow = head.next; let fast = head.next.next;

Solution

1. Step 1: Understand pointer initialization

   Slow pointer starts at head, fast pointer starts at head.next to allow fast to move two steps safely.

2. Step 2: Check pointer positions

   Starting fast at head.next prevents immediate equality and helps detect cycles correctly.

3. Final Answer:

   let slow = head; let fast = head.next; -> Option C

4. Quick Check:

   Slow at head, fast at head.next [OK]

Hint: Initialize slow at head and fast at head.next to detect cycles properly [OK]

Common Mistakes:

• Starting both pointers at head causing immediate equality
• Initializing slow pointer as null
• Starting pointers at incorrect nodes

3. Consider the following JavaScript code using Floyd's Algorithm to detect a cycle in a linked list:

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

// Linked list: 1 -> 2 -> 3 -> 4 -> 2 (cycle back to node with value 2)

What will be the output of hasCycle(head)?

medium

A. Runtime error

B. true

C. undefined

D. false

Solution

1. Step 1: Understand the linked list structure

   The list has a cycle because node with value 4 points back to node with value 2.

2. Step 2: Trace Floyd's Algorithm behavior

   Slow moves one step, fast moves two steps; they will meet inside the cycle, so function returns true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Cycle present = true [OK]

Hint: If pointers meet, cycle exists, so return true [OK]

Common Mistakes:

• Assuming no cycle returns true
• Confusing pointer equality with value equality
• Ignoring fast pointer null checks

4. The following code attempts to detect a cycle in a linked list using Floyd's Algorithm but has a bug:

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

What is the main issue with this code?

medium

A. It does not check if head is null before accessing next

B. It starts slow and fast pointers ahead, missing cycles at head

C. It compares values instead of nodes for equality

D. It moves slow pointer two steps instead of one

Solution

1. Step 1: Analyze pointer initialization

   Code sets slow = head.next and fast = head.next.next without checking if head or head.next exist.

2. Step 2: Identify potential runtime error

   If head is null or head.next is null, accessing next properties causes runtime error.

3. Final Answer:

   It does not check if head is null before accessing next -> Option A

4. Quick Check:

   Missing null check causes runtime error [OK]

Hint: Always check head and next before pointer initialization [OK]

Common Mistakes:

• Ignoring null checks causing runtime errors
• Starting pointers incorrectly but missing null checks
• Confusing node equality with value equality

5. You have a linked list where a cycle exists, but you want to find the node where the cycle begins using Floyd's Algorithm. Which additional step should you perform after detecting the cycle?

hard

A. Reset one pointer to head, then move both pointers one step at a time until they meet

B. Continue moving fast pointer two steps and slow pointer one step until fast reaches null

C. Reverse the linked list and check for repeated nodes

D. Count the number of nodes in the list and divide by two

Solution

1. Step 1: Detect cycle using Floyd's Algorithm

   When slow and fast pointers meet, a cycle is confirmed.

2. Step 2: Find cycle start node

   Reset slow pointer to head, then move slow and fast one step at a time; their meeting point is the cycle start.

3. Final Answer:

   Reset one pointer to head, then move both pointers one step at a time until they meet -> Option A

4. Quick Check:

   Reset and move pointers to find cycle start [OK]

Hint: Reset slow to head, move both pointers stepwise to find cycle start [OK]

Common Mistakes:

• Continuing fast pointer movement after cycle detection
• Trying to reverse list to find cycle start
• Using node count to find cycle start incorrectly