Evaluating Linked Lists
Types of Linked Lists
Singly Linked List
- Structure: Each node has data and a pointer to the next node.
- Operations:
- Insertion: O(1) at head; O(n) at tail (unless a tail pointer exists).
- Deletion: O(1) at head; O(n) elsewhere (need previous node).
- Traversal: O(n), must visit each node in sequence.
- Search: O(n), full traversal in worst case.
Doubly Linked List
- Structure: Each node has data, previous pointer, and next pointer.
- Operations:
- Insertion/Deletion: O(1) at head or tail; O(1) in middle if node reference is known.
- Traversal: O(n) forward or backward.
- Search: O(n), same as singly.
- Benefit: Easier bidirectional traversal and deletion without needing previous node.
Circular Linked List
- Structure: Last node points back to the first node, forming a loop.
- Operations:
- Insertion/Deletion: O(1) at head or tail.
- Traversal: O(n), but continues indefinitely without careful stopping conditions.
- Search: O(n), as with other linked lists.
- Benefit: Useful for round-robin scheduling or applications requiring continuous cycling.
Advantages and Disadvantages of Linked Lists
Advantages
- Dynamic Size: Can grow/shrink at runtime (unlike arrays).
- Efficient Insertions/Deletions: No shifting; just update pointers (O(1) if node known).
- Memory Utilization: Allocates memory on demand; avoids wasted capacity.
- Flexibility: Can adapt into doubly or circular lists.
Disadvantages of Linked Lists
- Access Time: O(n); no direct indexing (arrays have O(1) access).
- Memory Overhead: Extra space for pointers.
- No Random Access: Poor choice if frequent indexed access is required.
- Complexity: Pointer management can cause bugs, leaks, or dangling pointers.
Hint
When to Use Linked Lists
- When dataset size changes frequently.
- When frequent insertions/deletions occur (especially mid-list).
- When memory utilization is important and pointer overhead is acceptable.
| Aspect | Linked List | Array |
|---|---|---|
| Memory Utilization | Allocates dynamically | Fixed upfront size (may waste space) |
| Insert/Delete | O(1) (if node known) | O(n) (shifting needed) |
| Access | O(n), sequential | O(1), direct indexing |
| Flexibility | Can adapt (doubly, circular) | Fixed structure |
Tip
- When comparing with arrays, always mention access time (O(1) vs O(n)) and insertion/deletion efficiency.
- In code/diagram questions, clearly show pointer updates (e.g., for insertion/deletion).
Visualizing Linked List Operations
- Insertion (Singly): Update previous node’s pointer → new node.
- Deletion (Doubly): Link prev and next nodes → bypass deleted node.
- Traversal (Circular): Continue until reaching head again.
Common Mistake
- Thinking linked lists always have O(1) insertion: only true if node reference (or tail pointer) is available.
- Forgetting that traversal is required to access arbitrary elements (no direct indexing).
