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Implement Queue using Arrays

Easy

A circular buffer with two indices

Problem

Implement a queue (enqueue, dequeue, front) using an array (circular buffer).

In simple words

A line at a shop: join at the back, leave from the front.

The idea

Hold a front and a rear index and wrap both around with modulo, so dequeuing does not shift the whole array. Tracking the size separately is what lets you distinguish a full buffer from an empty one, since both leave the indices equal.

The trick

  • Wrap with `(i + 1) % capacity`.
  • Keep an explicit size or leave one slot empty; otherwise full and empty look identical.
  • Every operation is O(1).

This one walks through the worked example rather than tracing the algorithm frame by frame — a full walkthrough is still to be drawn. The code and the idea below are the real solution.

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Step 1 of 2. Here's the example — enqueue 1,2; dequeue Values: 1, 2.

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Optimal
timeO(1)spaceO(n)
1use front, rear indices modulo capacity2enqueue: arr[rear]=x; rear=(rear+1)%cap3dequeue: x=arr[front]; front=(front+1)%cap

Input

array
[1, 2]

Output

answer
—

Check yourself

2 quick questions about this walkthrough. A wrong answer costs nothing.

Example

Input:
enqueue 1,2; dequeue
Output:
returns 1

Practice this problem:GeeksforGeeks(opens in a new tab)

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