DSA interview question

Implement a Queue Using an Array (Circular Buffer)

Easy · Data structures · Topics: queue, array

The problem

A logging agent buffers outgoing log lines in a fixed-size array before shipping them, and it needs a first-in, first-out queue: enqueue at the back, dequeue from the front, both O(1). The naive design (dequeue by shifting everything left) is O(n), and "just advance a front index" wastes the freed slots at the start forever. A circular buffer fixes both: keep a head index and a count, and wrap positions around with modulo so freed slots are reused.

Example: enqueue(2), enqueue(3), dequeue() → 2, enqueue(4), dequeue() → 3.

Function signature

class ArrayQueue {
public:
    explicit ArrayQueue(int capacity);
    bool enqueue(int x);   // false if full
    int dequeue();         // -1 if empty
    int front() const;     // -1 if empty
    bool isEmpty() const;
    int size() const;
};

Test cases

Operations	Output	Explanation
`enqueue(2) enqueue(3) dequeue() enqueue(4) dequeue()`	`2 3`	First in, first out
`dequeue()` on an empty queue	`-1`
capacity 2: `enqueue(1) enqueue(2) enqueue(3)`	`true true false`	Full
capacity 3: `enqueue 1, 2, 3; dequeue; enqueue 4; dequeue ×3`	`1 2 3 4`	4 wraps into the freed slot 0
`enqueue(9) front() size()`	`9 1`

(Confirm: all operations O(1); capacity is fixed; −1 signals "empty".)

How interviewers phrase it

"Implement a queue using an array, with enqueue, dequeue, front and isEmpty" (GeeksforGeeks).
A circular queue with a fixed capacity is LeetCode 622.

Clarifying questions to ask

Fixed capacity? What should happen when it's full?
What should dequeue return when the queue is empty?
Is O(1) required for every operation? (Yes, which rules out shifting.)

Follow-ups to expect

"Why not shift elements on dequeue?"
"Why a count instead of just front and rear?"
"Where does the next element go?"
"How would you grow it?"
"Complexity?"

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