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Queue (FIFO)

Queue (FIFO)

First In, First Out.

Operations

Enqueue ’ O(1)
Dequeue ’ O(1)

Example Flow

enqueue(10) ' [10]
enqueue(20) ' [10, 20]
enqueue(30) ' [10, 20, 30]
dequeue() ' [20, 30]

Optimized Implementation

class Queue {
  constructor() {
    this.items = {};
    this.front = 0;
    this.rear = 0;
  }

  enqueue(value) {
    this.items[this.rear++] = value;
  }

  dequeue() {
    if (this.isEmpty()) return null;
    const value = this.items[this.front];
    delete this.items[this.front++];
    return value;
  }

  isEmpty() {
    return this.front === this.rear;
  }
}

Real-World Uses

Task scheduling
Printer queue
BFS
Request handling

Stack (LIFO) — FIFO’s counterpart, used for DFS and expression evaluation
Graphs — BFS traversal uses a queue for level-by-level exploration