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GATE | GATE-CS-2016 (Set 2) | Question 25

  • Difficulty Level : Hard
  • Last Updated : 28 Jun, 2021

N items are stored in a sorted doubly linked list. For a delete operation, a pointer is provided to the record to be deleted. For a decrease-key operation, a pointer is provided to the record on which the operation is to be performed. An algorithm performs the following operations on the list in this order:
Θ(N) delete, O(log N) insert, O(log N) find, and Θ(N) decrease-key

What is the time complexity of all these operations put together
(A) O(Log2N)
(B) O(N)
(C) O(N2)
(D) Θ(N2 Log N)

Answer: (C)

Explanation: The time complexity of decrease-key operation is Θ(1) since we have the pointer to the record where we have to perform the operation. However, we must keep the doubly linked list sorted and after the decrease-key operation we need to find the new location of the key. This step will take Θ(N) time and since there are Θ(N) decrease-key operations, the time complexity becomes O(N²).

Note that the other three operations have a lower bound than this one.

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