Top 30 Most Common Linked List Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Linked list interview questions are a staple in technical interviews, particularly for software engineering roles. Mastering these questions demonstrates a strong foundation in data structures and algorithms, essential skills for problem-solving and efficient coding. Understanding linked lists, their operations, and variations is crucial for tackling more complex problems during interviews.
What Are linked list interview questions?
linked list interview questions cover a range of topics related to this fundamental data structure. They assess your understanding of what a linked list is, how it differs from other structures like arrays, and how to perform common operations such as insertion, deletion, traversal, and reversal. Questions often involve problem-solving scenarios requiring you to manipulate linked lists efficiently, identify cycles, find specific nodes, or merge/sort lists. Preparing for linked list interview questions helps solidify your knowledge of pointers and dynamic memory allocation.
Why Do Interviewers Ask linked list interview questions?
Interviewers ask linked list interview questions for several reasons. Firstly, they test your foundational computer science knowledge and your ability to work with abstract data structures. Secondly, linked list problems often require careful pointer manipulation, revealing your attention to detail and ability to handle complex logic. Thirdly, they assess your problem-solving approach and algorithmic thinking, especially for questions involving optimization (like single-pass solutions). Proficiency with linked lists indicates you can handle dynamic data and understand memory management implications, skills vital for many programming tasks.
What is a linked list? Explain its types.
What are the advantages of using a linked list over arrays?
How do you find the length of a singly linked list?
How to find the middle element of a singly linked list in one pass?
How to reverse a singly linked list?
How to detect a cycle in a linked list?
How to delete a node from a linked list?
How to merge two sorted linked lists?
How to remove duplicates from a linked list?
How to implement a function to check if the linked list is sorted?
What is the difference between singly and doubly linked lists?
Write code to print all nodes of a linked list.
How to insert a node at the beginning of a linked list?
How to insert a node at the end of a linked list?
How to reverse nodes of a linked list in groups of k?
How to find the nth node from the end of a linked list?
How to swap every two nodes in a linked list?
How to find if two linked lists intersect?
What is the difference between an array and linked list?
How to delete alternate nodes in a linked list?
What is a circular linked list and where is it used?
How to detect and remove a cycle in a linked list?
How to implement a stack or queue using a linked list?
How to find the frequency of a given element in a linked list?
How do you copy a linked list with a random pointer?
Explain the time complexity to access the kth element.
How to implement a linked list using arrays?
How to merge two sorted linked lists recursively?
Explain the difference between a linked list and a binary tree.
How to implement a function to detect palindrome in a linked list?
Preview List
1. What is a linked list? Explain its types.
Why you might get asked this:
This is a fundamental question to check your basic knowledge of data structures and their variations. It assesses your understanding of the core concept before diving into complex problems.
How to answer:
Define a linked list as connected nodes, each containing data and a pointer. Explain the node structure and list the main types: singly, doubly, and circular, briefly describing each.
Example answer:
A linked list is a linear data structure where nodes hold data and a reference to the next node. Nodes aren't contiguous. Types: Singly (next pointer), Doubly (prev+next), Circular (last points to head), Circular Doubly.
2. What are the advantages of using a linked list over arrays?
Why you might get asked this:
This question tests your understanding of when to use linked lists by comparing their characteristics against arrays, highlighting trade-offs and benefits.
How to answer:
Focus on dynamic size, ease of insertion/deletion (no shifting), efficient memory use, and non-contiguous storage as key advantages.
Example answer:
Linked lists offer dynamic size and efficient insertions/deletions (O(1) if node known), unlike arrays which are fixed-size and require O(n) for shifting elements during insert/delete. They use non-contiguous memory.
3. How do you find the length of a singly linked list?
Why you might get asked this:
A basic traversal problem that checks your ability to iterate through a linked list correctly and handle the end condition.
How to answer:
Describe the iterative process of starting from the head, moving to the next node until null, and counting each node visited.
Example answer:
Start a counter at 0. Begin at the head node. While the current node is not null, increment the counter and move to the next node using its 'next' pointer. The final count is the length.
4. How to find the middle element of a singly linked list in one pass?
Why you might get asked this:
This common problem tests your ability to use multiple pointers for efficient traversal, specifically the fast and slow pointer technique.
How to answer:
Explain using two pointers, slow and fast. Slow moves one step, fast moves two. When fast reaches the end (or null), slow is at the middle.
Example answer:
Use two pointers: slow and fast. Initialize both to head. Move slow one step and fast two steps at a time. When fast reaches the end (or fast->next is null), slow will be pointing to the middle node of the list.
5. How to reverse a singly linked list?
Why you might get asked this:
A core linked list manipulation problem requiring careful pointer handling. It assesses your ability to update links without losing track of nodes.
How to answer:
Explain the iterative method using three pointers: previous, current, and next to update the 'next' pointer of each node to point backward.
Example answer:
Use 'prev' (initially null), 'curr' (initially head), and 'next' pointers. Iterate: store 'curr->next' in 'next', set 'curr->next = prev', move 'prev = curr', move 'curr = next'. Update head to 'prev' at the end.
6. How to detect a cycle in a linked list?
Why you might get asked this:
Tests your knowledge of cycle detection algorithms, particularly Floyd's Cycle-Finding Algorithm, which is a standard technique.
How to answer:
Describe Floyd's algorithm: use a slow pointer (1 step) and a fast pointer (2 steps). If they meet at any point, a cycle exists.
Example answer:
Use Floyd's Cycle-Finding Algorithm. Employ two pointers, slow and fast, starting at the head. Slow moves one node at a time, fast moves two nodes. If they ever meet, there is a cycle in the linked list.
7. How to delete a node from a linked list?
Why you might get asked this:
Evaluates your ability to modify the list structure by updating pointers correctly to bypass and remove a node.
How to answer:
Explain finding the node's predecessor and updating its 'next' pointer to skip the node to be deleted. Handle edge cases like deleting the head or a node given only the node pointer.
Example answer:
To delete a node, find its preceding node. Update the predecessor's 'next' pointer to point to the node after the one to be deleted. Free the memory of the deleted node. Handle head deletion separately.
8. How to merge two sorted linked lists?
Why you might get asked this:
A classic problem combining traversal and sorting logic. It tests your ability to build a new list or modify existing pointers based on sorted order.
How to answer:
Explain iteratively comparing the heads of both lists, adding the smaller node to a new merged list, and advancing the pointer of the list from which the node was taken.
Example answer:
Iteratively compare the current nodes of both sorted lists. Append the node with the smaller value to a new merged list and advance its pointer. Once one list is exhausted, append the remaining nodes of the other list.
9. How to remove duplicates from a linked list?
Why you might get asked this:
Tests your use of auxiliary data structures (like hash sets) for efficient lookup or your ability to handle duplicates with limited space.
How to answer:
Explain using a hash set to store encountered node values. Iterate through the list; if a value is in the set, delete the current node; otherwise, add the value to the set and move on.
Example answer:
Use a hash set to keep track of values seen so far. Traverse the list. For each node, check if its data is in the set. If yes, remove the node. If no, add the data to the set and move to the next node.
10. How to implement a function to check if the linked list is sorted (ascending or descending)?
Why you might get asked this:
A simple traversal problem that checks your logical ability to compare adjacent elements based on a condition.
How to answer:
Traverse the list, comparing each node's data with the next node's data. If any pair violates the required sorted order (ascending/descending), return false. If the loop finishes, return true.
Example answer:
Iterate through the list from the head. Compare the current node's data with the next node's data. If current->data > current->next->data for ascending (or < for descending) at any point, return false. If loop completes, return true.
11. What is the difference between singly and doubly linked lists?
Why you might get asked this:
Checks your understanding of variations and their implications for operations like backward traversal or efficient deletion with only a node pointer.
How to answer:
Highlight the difference in pointers: singly has only 'next', doubly has 'next' and 'previous'. Explain how this affects traversal direction and certain operations.
Example answer:
A singly linked list node has only a 'next' pointer, allowing traversal in one direction. A doubly linked list node has both 'next' and 'previous' pointers, enabling traversal in both forward and backward directions.
12. Write code to print all nodes of a linked list.
Why you might get asked this:
A basic coding task to see if you can implement a simple traversal loop and access node data.
How to answer:
Provide pseudocode or actual code for traversing from the head to the end (null) and printing the data of each node visited.
Example answer:
This iterates and prints data until the end of the list is reached.
13. How to insert a node at the beginning of a linked list?
Why you might get asked this:
Tests a fundamental insertion operation at a specific position, requiring updating the head pointer.
How to answer:
Explain creating a new node, setting its 'next' pointer to the current head, and then updating the list's head pointer to point to the new node.
Example answer:
Create the new node. Set newNode->next = head. Then update head = newNode. This makes the new node the first element, pointing to the original list's head.
14. How to insert a node at the end of a linked list?
Why you might get asked this:
Tests another fundamental insertion operation, requiring traversal to the end before linking the new node.
How to answer:
Explain traversing the list to find the last node (where 'next' is null). Set the last node's 'next' pointer to the new node. Handle the case of an empty list.
Example answer:
Create the new node and set its 'next' to null. If the list is empty, set head to the new node. Otherwise, traverse to the last node (whose 'next' is null) and set its 'next' pointer to the new node.
15. How to reverse nodes of a linked list in groups of k?
Why you might get asked this:
A more challenging problem combining reversal logic with group management. Tests recursive thinking or careful iterative partition and linking.
How to answer:
Explain recursively or iteratively reversing the first k nodes, then recursively calling the function on the rest of the list and linking the reversed group to the result of the recursive call.
Example answer:
Reverse the first k nodes of the current list. Recursively call the function on the rest of the list starting from k+1 node. Attach the reversed k-group to the head returned by the recursive call. Handle remaining nodes if less than k.
16. How to find the nth node from the end of a linked list?
Why you might get asked this:
Another common problem using the two-pointer technique, specifically the ' відстань ' method to maintain a fixed gap.
How to answer:
Explain using two pointers, main and reference. Move the reference pointer 'n' steps ahead. Then move both pointers one step at a time until the reference pointer reaches the end. The main pointer will be at the nth node from the end.
Example answer:
Use two pointers: main and reference. Move reference pointer 'n' steps from head. Then move both main and reference one step until reference reaches null. Main pointer will then point to the nth node from the end of the list.
17. How to swap every two nodes in a linked list?
Why you might get asked this:
Tests pointer manipulation skills for rearranging nodes. Can be done by swapping data or, more complexly, by swapping the node pointers themselves.
How to answer:
Describe iterating through the list, processing nodes in pairs. For each pair, adjust the pointers of the previous node (or head), the two nodes being swapped, and the node after the pair.
Example answer:
Iterate through the list, keeping track of the previous node. Process pairs of nodes (current, next). Swap pointers: previous points to next, current points to next->next, next points to current. Update previous for the next pair.
18. How to find if two linked lists intersect?
Why you might get asked this:
Tests your ability to handle multiple lists and possibly optimize using length differences to align traversal.
How to answer:
Find the lengths of both lists. Calculate the difference in lengths. Advance the pointer of the longer list by the difference. Then traverse both lists simultaneously. If pointers meet, that's the intersection node.
Example answer:
Calculate lengths of both lists. Find the difference d. Advance the head of the longer list by d nodes. Then traverse both lists node by node simultaneously. If they point to the same node at any point, they intersect there.
19. What is the difference between an array and linked list?
Why you might get asked this:
A fundamental comparison question to check your understanding of the core structural and performance differences between two basic linear data structures.
How to answer:
Contrast them based on size (fixed vs dynamic), memory allocation (contiguous vs non-contiguous), insertion/deletion efficiency, and access time (random access O(1) vs sequential access O(n)).
Example answer:
Arrays are fixed-size, contiguous memory blocks allowing O(1) random access but slow O(n) insert/delete. Linked lists are dynamic, non-contiguous nodes allowing O(1) insert/delete at ends/known pos but slow O(n) sequential access.
20. How to delete alternate nodes in a linked list?
Why you might get asked this:
A straightforward traversal and deletion problem that requires careful pointer updates to skip nodes correctly.
How to answer:
Traverse the list starting from the head. For the current node, update its 'next' pointer to point to the node after the next one (skipping the immediate next node). Then move the current pointer to the node it now points to.
Example answer:
Start at the head. Loop while the current node and the node after it are not null. Set currentNode->next = currentNode->next->next. Then move currentNode = currentNode->next. This effectively skips and disconnects the alternate node.
21. What is a circular linked list and where is it used?
Why you might get asked this:
Tests your knowledge of a specific linked list variation and its practical applications, showing awareness of different use cases.
How to answer:
Define a circular linked list where the last node points back to the head. Mention typical uses like buffering, task scheduling, or repeatedly cycling through a list of items.
Example answer:
A circular linked list is where the 'next' pointer of the last node points back to the head node, forming a circle. They're useful for implementing buffers, round-robin scheduling, or whenever you need a list that loops infinitely.
22. How to detect and remove a cycle in a linked list?
Why you might get asked this:
Combines cycle detection with the process of breaking the cycle, requiring logical steps after identification.
How to answer:
First, detect the cycle using Floyd's algorithm (slow/fast pointers). If a cycle exists, find the starting node of the loop. Then, traverse from the node before the loop start and set its 'next' pointer to null.
Example answer:
Detect the cycle using slow/fast pointers. If they meet, there's a cycle. To remove, find the cycle start (e.g., move slow to head, keep fast at meeting point, move both one step until they meet again - this is the start). Traverse from head, find the node whose 'next' is the cycle start, and set its 'next' to null.
23. How to implement a stack or queue using a linked list?
Why you might get asked this:
Tests your understanding of abstract data types (Stack/Queue) and how their operations (LIFO/FIFO) map onto linked list manipulations.
How to answer:
For a stack, explain using the head for both push (insert at head) and pop (delete at head). For a queue, explain enqueuing at the tail (insert at end) and dequeuing from the head (delete at head).
Example answer:
Stack (LIFO): Implement push by inserting nodes at the head (O(1)). Implement pop by deleting nodes from the head (O(1)). Queue (FIFO): Implement enqueue by inserting nodes at the tail (O(1) with tail pointer). Implement dequeue by deleting from the head (O(1)).
24. How to find the frequency of a given element in a linked list?
Why you might get asked this:
A basic traversal and counting problem, similar to finding length but with a condition. Checks iterative processing.
How to answer:
Describe traversing the list from the head. Maintain a counter. For each node, check if its data matches the target element. If it does, increment the counter. Continue until the end.
Example answer:
Initialize a frequency counter to 0. Start traversing from the head. For every node visited, compare its data with the target element. If they match, increment the counter. Continue until the end of the list (null). Return the final count.
25. How do you copy a linked list with a random pointer?
Why you might get asked this:
A complex problem requiring careful handling of both 'next' and 'random' pointers, often solved efficiently using an intermediate mapping or in-place cloning.
How to answer:
Explain the process: 1. Clone nodes and insert them after original nodes (A -> A' -> B -> B'...). 2. Assign random pointers for cloned nodes using original nodes' random pointers (A'.random = A.random.next). 3. Separate the original and cloned lists by restoring original pointers and creating the new list's pointers.
Example answer:
Clone each node and insert it right after its original (A->A'->B->B'...). Set random pointers for cloned nodes: A'.random = A.random->next. Then, separate lists: original A->B..., cloned A'->B'... by fixing 'next' pointers.
26. Explain the time complexity to access the kth element in a linked list.
Why you might get asked this:
Tests your understanding of linked list performance characteristics, specifically that access is sequential, unlike arrays.
How to answer:
State that accessing the kth element requires traversing from the beginning up to the kth node. Thus, the time complexity is proportional to k, which is O(k) in the worst case (accessing the last element is O(n)).
Example answer:
Accessing the kth element requires starting from the head and traversing k-1 steps. In the worst case (accessing the last element), this takes O(n) time, where n is the length of the list. Therefore, it's O(k).
27. How to implement a linked list using arrays?
Why you might get asked this:
Tests your understanding of the core concepts of linked lists (nodes, pointers) by asking you to simulate them using a different data structure.
How to answer:
Explain using two arrays: one for data, one for the index of the next node. Use integer indices as 'pointers'. Manage free spaces using an "available list" or similar method.
Example answer:
You can use two arrays, data[] and next[]. data[i] stores the value of node i, and next[i] stores the index of the next node. Use a special index (e.g., -1) for null. Manage unused indices via a free list.
28. How to merge two sorted linked lists recursively?
Why you might get asked this:
Tests recursive problem-solving skills applied to linked lists and merging sorted structures.
How to answer:
Explain the base cases (if either list is null). For the recursive step, compare heads: take the smaller node, set its 'next' to the result of recursively merging the rest of its list with the other list, and return the smaller node.
Example answer:
Base case: If either list is null, return the other list. Recursive step: If list1.data <= list2.data, set list1.next = mergeLists(list1.next, list2) and return list1. Otherwise, set list2.next = mergeLists(list1, list2.next) and return list2.
29. Explain the difference between a linked list and a binary tree.
Why you might get asked this:
Tests your understanding of how data structures differ fundamentally in their connection patterns (linear vs hierarchical) and typical use cases.
How to answer:
Highlight the structural difference: linked list nodes point linearly (one or two directions), while binary tree nodes have a hierarchical structure (parent to child, each parent having at most two children). Mention they solve different problems.
Example answer:
A linked list is a linear structure where each node points to the next (and possibly previous), forming a sequence. A binary tree is a hierarchical structure where each node points to at most two children, representing relationships like parent-child or used for searching/sorting.
30. How to implement a function to detect palindrome in a linked list?
Why you might get asked this:
A challenging problem requiring comparison of the first half with the reversed second half. Tests multiple techniques: finding middle, reversing, comparison, and potentially restoring the list.
How to answer:
Explain finding the middle using fast/slow pointers. Reverse the second half of the list starting from the middle's next node. Compare the first half with the reversed second half element by element. Optionally, reverse the second half back to restore the original list structure.
Example answer:
Find the middle of the list using slow/fast pointers. Reverse the linked list from the node after the middle to the end. Compare elements of the original first half with the reversed second half. If they match throughout, it's a palindrome.
Other Tips to Prepare for a linked list interview questions
Mastering linked list interview questions goes beyond memorizing answers; it requires deep understanding and practice. Start by implementing basic operations like insertion, deletion, and traversal from scratch in your preferred programming language. "The only way to learn a new programming language is by writing programs in it," a principle that applies equally to mastering data structures. Solve problems on platforms like LeetCode or HackerRank focusing specifically on linked lists. Practice explaining your logic out loud as you code; this is crucial for the interview setting. Use tools like Verve AI Interview Copilot to simulate interview conditions and get feedback on your approach and explanation style for linked list interview questions. Verve AI Interview Copilot can provide targeted practice sessions helping you become more comfortable articulating complex solutions. Remember, consistent practice is key. As the saying goes, "Practice makes permanent." Leverage resources like https://vervecopilot.com for structured practice tailored to common linked list interview questions and other technical topics. Regularly reviewing different types of linked lists and their specific operations will also bolster your confidence.
Frequently Asked Questions
Q1: What are the typical use cases for linked lists?
A1: Used for implementing stacks/queues, adjacency lists in graphs, dynamic memory allocation, and when frequent insertions/deletions are needed.
Q2: Are linked lists stored contiguously in memory?
A2: No, nodes can be scattered anywhere in memory. Pointers are used to link them together.
Q3: What is a sentinel node?
A3: A dummy node (often at head or tail) used to simplify list operations by avoiding edge cases like empty lists.
Q4: What is the space complexity of linked lists?
A4: O(n) because you store n nodes, each requiring space for data and a pointer(s).
Q5: When is an array a better choice than a linked list?
A5: When you need frequent random access (indexing), when the size is fixed or known beforehand, or when memory locality is important for performance.
Q6: Can a linked list have multiple pointers per node?
A6: Yes, a doubly linked list has two (next and previous), and some advanced structures might have more (e.g., random pointers).
