GATE CSE 2022 Q62: Minimum enqueues on Q1 to reverse its elements into Q2

Programming and Data Structures
2 marks
NAT
No negative marking

Consider the queues $Q_1$ containing four elements and $Q_2$ containing none (shown as the Initial State in the figure). The only operations allowed on these two queues are Enqueue(Q,element) and Dequeue(Q). The minimum number of Enqueue operations on $Q_1$ required to place the elements of $Q_1$ in $Q_2$ in reverse order (shown as the Final State in the figure) without using any additional storage is___________.

Initial state: Q1 holds 1, 2, 3, 4 from the head and Q2 is empty. Final state: Q1 is empty and Q2 holds 4, 3, 2, 1 from the head — Tap the figure to enlarge

Try it: numerical answer: type the value

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Numerical answer. No negative marking.

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Answer: 0 (official GATE 2022 final answer key)

Concept used: Only enqueues on $Q_1$ are counted; $Q_2$ itself can be rotated (dequeue then enqueue into $Q_2$ ) to place each new element at its front.

First step: Move 1 into $Q_2$ ; move 2 in and rotate $Q_2$ once so 2 is at the head; continue with 3 and 4.

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Source: official GATE 2022 CS question paper and final answer key, organised by IIT Kharagpur. Question paper · Answer key

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GATE CSE 2022 Q62: Minimum enqueues on Q1 to reverse its elements into Q2

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