Design Linked List

Design Linked List

What is this problem about?

The Design Linked List coding problem is a fundamental data structure task. You are required to implement a singly or doubly linked list from scratch. The implementation should support common operations: getting a value at a specific index, adding a node at the head, adding a node at the tail, adding a node before a specific index, and deleting a node at a specific index.

Why is this asked in interviews?

Microsoft, Amazon, and Meta frequently use this to verify a candidate's grasp of pointer manipulation and manual memory management. It’s a "back-to-basics" question that exposes whether you truly understand how higher-level abstractions work under the hood. It tests your ability to handle null checks, maintain the "size" of a collection, and manage edge cases like deleting the only node in a list or inserting at the very end.

Algorithmic pattern used

The Linked List interview pattern relies on pointer/reference manipulation. For a singly linked list, each node points to the next node. For a doubly linked list, nodes point to both next and prev. You typically maintain a head pointer and sometimes a tail pointer and a size variable to optimize operations like addAtTail.

Example explanation

Let's build a simple list:

1. addAtHead(1): Head -> [1] -> Null. Size = 1.
2. addAtTail(3): Head -> [1] -> [3] -> Null. Size = 2.
3. addAtIndex(1, 2): We want to insert '2' at index 1. The system traverses to index 0, makes the new node '2' point to what index 0 was pointing to (3), and then makes index 0 point to '2'. List: 1 -> 2 -> 3.
4. deleteAtIndex(1): The node at index 0 is made to point to index 2 (bypassing index 1). List: 1 -> 3.

Common mistakes candidates make

• Off-by-one errors: Especially during index-based traversal or deletion.
• Null Pointer Exceptions: Failing to check if curr.next is null before accessing its properties.
• Edge Cases: Not properly handling head or tail updates when the list is empty or has only one element.
• Memory Leaks: In languages like C++, forgetting to free the memory of a deleted node.

Interview preparation tip

Practice implementing both singly and doubly linked lists. Doubly linked lists are harder to code but make deletions at a known node much faster ($O(1)$). Always use a "dummy" or "sentinel" head node to simplify your logic—it eliminates many special cases for inserting or deleting at the very beginning.