Top 10 Data Structure Interview Questions on Linked Lists

Introduction

Linked lists are a fundamental data structure that every aspiring software engineer should understand in depth. They are especially important to master for coding interviews, where linked list problems are extremely common.

In this post, we‘ll cover the top 10 most frequently asked linked list interview questions, including different types of linked lists, key operations, comparisons to arrays, and more. Whether you are preparing for an upcoming interview or just brushing up on core data structure concepts, this guide will deepen your understanding of linked lists and set you up for success. Let‘s dive in!

What is a linked list?

A linked list is a linear data structure similar to an array, but instead of elements being stored contiguously in memory, each element is stored in a separate node. Nodes contain both the element data and a reference (or link) to the next node in the sequence.

This allows for efficient insertion and deletion of elements, as only the links between nodes need to be updated. However, random access of elements is not possible like it is with arrays – to access an element, you must traverse the linked list from the beginning until you reach the desired node.

Types of linked lists

There are four main types of linked lists:

Singly linked list

A singly linked list is unidirectional – each node only contains a reference to the next node in the sequence. The last node points to null to indicate the end of the list.

A node in a singly linked list is typically represented like:

struct Node {
int data;
Node* next;
};

Traversal of a singly linked list must be done in a forward direction starting from the head node. There is no way to directly access previous elements.

Doubly linked list

In a doubly linked list, each node contains references to both the next and previous nodes. This allows traversal in both forward and backward directions, but requires extra space to store the previous node pointer.

A doubly linked list node is represented as:

struct Node {
int data;
Node prev;
Node next;
};

The two ends of a doubly linked list are typically referred to as the head (first node) and tail (last node). The previous pointer of the head and the next pointer of the tail both point to null.

Circular singly linked list

A circular singly linked list is similar to a singly linked list, except the last node, instead of pointing to null, points back to the first node, forming a loop or circle.

Traversal of a circular singly linked list can begin at any node, and following the next pointers will eventually loop back around to the starting node. A pointer to the last node gives quick access to both the beginning and end of the list.

Circular doubly linked list

A circular doubly linked list is like a doubly linked list, but the previous pointer of the head node points to the tail, and the next pointer of the tail points to the head, forming a circular loop in both directions.

Converting a binary tree to a doubly linked list

A common linked list interview question involves converting a binary tree into a doubly linked list, in-place.

The idea is to perform an inorder traversal of the binary tree. We initialize the previous node as null before calling the recursive function. For each node visited, we set its left child to the previous node, and the previous node‘s right child to the current node (if previous is not null). Then we update the previous node to the current node and recursively convert the right subtree.

After the traversal, the left pointer of the head will be null, and the right pointer of the tail will point to the head, forming a circular doubly linked list.

Here is the C++ code:

void convertToDLL(Node* root, Node head, Node tail) {
if (root == NULL) return;

convertToDLL(root->left, head, tail);

if (tail == NULL) {
head = root;
} else {
root->left = tail;
(tail)->right = root;
}

*tail = root;

convertToDLL(root->right, head, tail);
}

Key operations on linked lists

The three primary operations performed on linked lists are insertion of a new node, deletion of a node, and traversal of the list.

Insertion

To insert a new node into a linked list, you must first allocate space for the new node and assign its data value. Then, you set the new node‘s next pointer to the desired position and update the next pointer of the previous node to point to the new node.

Insertion can be done at the beginning of the list (pushing to the head), the end of the list (appending to the tail), or anywhere in the middle.

Time complexity of insertion is O(1) for pushing and appending, and O(n) for inserting in the middle, as we must traverse to the insertion point first.

Deletion

To delete a node, you simply update the next pointer of the previous node to skip over the node to be deleted. For doubly linked lists, you must update the previous pointer of the next node as well.

Deletion can occur at the beginning (pop head), end (remove tail), or middle of the list.

Like insertion, time complexity is O(1) for head and tail removal, and O(n) for deleting a middle node due to the traversal required to reach it.

Traversal

Traversing a linked list simply means visiting each node in order from the head to the tail (or vice versa for a doubly linked list). This is typically done using a loop and a pointer that starts at the head and follows the next pointers until reaching null (or the head again for a circular list).

The time complexity of traversal is O(n) as each node must be visited once.

Merge sort for linked lists

Merge sort is the most efficient sorting algorithm for linked lists, with a time complexity of O(n log n). It is a divide-and-conquer algorithm that recursively splits the list into sublists until each sublist contains only one node, and then merges the sublists back together in sorted order.

Merge sort is well-suited to linked lists because the merge operation can be performed efficiently by rearranging pointers, without the need for extra space. It is also a stable sort, meaning the relative order of equal elements is preserved.

Linked lists vs arrays

Linked lists and arrays are both used to store linear collections of elements, but they have different strengths and weaknesses.

Arrays provide O(1) random access to elements and are easy to iterate through, but their size is fixed at creation time and inserting or deleting elements in the middle is expensive, requiring shifting all subsequent elements.

Linked lists have efficient O(1) insertion and deletion at any position, and their size can easily be changed, but they do not offer random access to elements and require extra space for the node pointers.

In general, linked lists are preferred over arrays when:

• Data size is unknown or may change
• Insertion/deletion in the middle is frequent
• Random access is not needed

And arrays are a better choice when:

• Size of data is fixed and known ahead of time
• Random access of elements is required
• Extra space usage needs to be minimized

Inserting a node in the middle of a singly linked list

One of the most basic linked list coding exercises is inserting a node into the middle of a singly linked list, given only a pointer to the node after which the new node should be inserted.

To do this, we first allocate the new node and set its data value. We then set the new node‘s next pointer to the next node of the given pointer. Finally, we update the given node‘s next pointer to point to the new node.

void insertAfter(Node* prev_node, int new_data) {
if (prev_node == NULL) return;

Node* new_node = new Node();
new_node->data = new_data;
new_node->next = prev_node->next;
prev_node->next = new_node;
}

Only two pointer changes are required: the next pointer of the new node and the next pointer of the previous node. The time complexity is O(1) since no traversal is needed.

Pros and cons of linked lists

Advantages of linked lists:

• Dynamic size that can easily be resized
• Efficient insertion and deletion of nodes
• Efficient memory utilization – space is only allocated as needed
• Flexibility – they can be used to implement stacks, queues, graphs, etc.

Disadvantages of linked lists:

• No random access of elements
• Extra space required for node pointers
• Not cache-friendly, as nodes are not contiguous in memory
• Traversal is slower than arrays due to lack of locality of reference

Conclusion

Linked lists are a versatile and essential data structure to understand for aspiring software engineers and coding interviews. They efficiently solve many problems requiring insertion, deletion, and dynamic sizing that arrays can‘t handle well.

To master linked lists, it‘s important to understand the different types (singly, doubly, circular), their tradeoffs with arrays, and common operations like insertion, deletion, traversal, and sorting. Practicing manipulating linked list pointers until it becomes second nature will help immensely in interviews.

While coding interview questions can be challenging, with practice and a solid grasp of these core linked list concepts, you‘ll be well-prepared to handle anything they throw at you. Just remember – think carefully about edge cases, walk through your code line-by-line, and don‘t be afraid to use the hints if you get stuck. You‘ve got this!