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Enqueue

Overview​

Enqueue is the fundamental operation of a Queue that adds an element to the rear (end) of the queue. It preserves the First In, First Out (FIFO) ordering by ensuring new elements join the line at the back.

What Does Enqueue Do?​

The enqueue operation:

  • Inserts a new element
  • Places it at the rear of the queue
  • Updates internal pointers or indices

No existing elements change their relative order.

How It Works​

Depending on the implementation:

Array-based Queue​

  • Insert at the rear index
  • Increment the rear pointer
  • May wrap around in a circular queue

Linked-list-based Queue​

  • Create a new node
  • Link it after the current rear
  • Update the rear reference

Example​

Pseudocode​

enqueue(queue, value):
    if queue is full:
        error "Queue Overflow"
    rear = rear + 1
    queue[rear] = value

Real-world Analogy​

Enqueue is like joining the end of a line. You don’t cut in, you take your place at the back 🚶‍♂️➡️🚶‍♀️.

Time Complexity​

  • Time: O(1)
  • Space: O(1) per operation

Error Conditions​

  • Queue overflow: when the queue is full (in fixed-size implementations)

Use Cases​

  • Adding tasks to a scheduler
  • Receiving network packets
  • Job submission systems
  • Event handling pipelines

Advantages​

  • Fast insertion
  • Preserves ordering
  • Simple operation

Limitations​

  • Cannot insert at arbitrary positions
  • Depends on queue capacity (if fixed)