How would you implement a memory-efficient queue using a singly linked list with head and tail pointers, and what are the time complexities of enqueue and dequeue?

• A. Maintain head and tail; enqueue O(1), dequeue O(1)
• B. Use only a head pointer; both operations O(n)
• C. Use a circular buffer; both O(1) average
• D. Rebuild the list on every operation; O(n)

Answer: (D)

When you aim for minimal extra state in a queue built from a singly linked list, you would avoid keeping extra pointers beyond what’s strictly needed. Rebuilding the list on every operation achieves that: you don’t store a persistent tail pointer or other helper data, so the memory footprint stays small. The trade-off is time. To perform each enqueue or dequeue, you must reconstruct or relink the entire list, so every operation touches all elements. That makes both enqueue and dequeue run in linear time, O(n). In this light, the described approach aligns with memory efficiency at the cost of slower operations. If you did keep a head and tail pointer, you’d get O(1) for both operations, but you’d be using a tiny amount more space for that extra pointer.