GATE CS 2018 Set 1 — Question 13
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Programming & Data Structures → Linked Lists → Queue & Circular Queue
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Question
A queue is implemented using a non-circular singly linked list. The queue has a head pointer and a tail pointer, as shown in the figure. Let denote the number of nodes in the queue. Let 
Which one of the following is the time complexity of the most time-efficient implementation of
enqueue be implemented by inserting a new node at the head, and dequeue be implemented by deletion of a node from the tail.enqueue and dequeue, respectively, for this data structure?Correct answer
(B) θ(1), θ(n)
Solution
- Enqueue at the head: To insert a new node at the head of a singly linked list, we create the node, set its
nextpointer to the currenthead, and update theheadpointer. This operation does not depend on the number of nodes , so its time complexity is . - Dequeue from the tail: To delete a node from the tail of a singly linked list, we need to update the
nextpointer of the second-to-last node toNULLand update thetailpointer. Since it's a singly linked list, we don't have a direct pointer to the predecessor of the tail. We must traverse the list from theheadto find the -th node. This traversal takes time proportional to , so the time complexity is .
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