Enqueue Operation in DSA C# - Time & Space Complexity

Time Complexity: Enqueue Operation

O(1)

Understanding Time Complexity

We want to understand how the time needed to add an item to a queue changes as the queue grows.

How does the enqueue operation behave when the queue gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class Queue
{
    private List<int> items = new List<int>();

    public void Enqueue(int item)
    {
        items.Add(item); // Add item to the end
    }
}
    

This code adds a new item to the end of a queue implemented using a list.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Adding an item to the end of the list.
• How many times: Once per enqueue call, no loops involved.

How Execution Grows With Input

Adding one item takes about the same time regardless of queue size.

Input Size (n)	Approx. Operations
10	1 add operation
100	1 add operation
1000	1 add operation

Pattern observation: The time to add does not increase as the queue grows; it stays constant on average.

Final Time Complexity

Time Complexity: O(1) on average

This means adding an item to the queue takes the same amount of time on average no matter how many items are already in it.

Common Mistake

[X] Wrong: "Adding an item takes longer as the queue grows because the list gets bigger."

[OK] Correct: The list adds items at the end without scanning or shifting, so the time stays the same on average regardless of size, though occasional resizing can take longer.

Interview Connect

Understanding that enqueue is a constant time operation on average helps you explain efficient queue usage clearly and confidently.

Self-Check

"What if the queue was implemented using a linked list instead of a list? How would the time complexity change?"

Practice

1. What does the Enqueue operation do in a queue data structure?

easy

A. Adds an element to the end of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand queue behavior

   A queue works like a line where new people join at the end.

2. Step 2: Define enqueue operation

   Enqueue means adding a new element at the back of the queue.

3. Final Answer:

   Adds an element to the end of the queue -> Option A

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue means add at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to enqueue an integer value 10 into a Queue<int> named q in C#?

easy

A. q.Enqueue(10);

B. q.Add(10);

C. q.Push(10);

D. q.Insert(10);

Solution

1. Step 1: Recall C# Queue method

   The method to add elements in Queue<T> is Enqueue.

2. Step 2: Match syntax

   Calling q.Enqueue(10); adds 10 to the queue correctly.

3. Final Answer:

   q.Enqueue(10); -> Option A

4. Quick Check:

   Use Enqueue() to add in C# Queue [OK]

Hint: Use Enqueue() method to add elements in C# Queue [OK]

Common Mistakes:

• Using Add() which is not in Queue
• Using Push() which is for stacks
• Using Insert() which is not a Queue method

3. What will be the output of the following C# code?

Queue<string> q = new Queue<string>();
q.Enqueue("apple");
q.Enqueue("banana");
q.Enqueue("cherry");
Console.WriteLine(string.Join(", ", q));

medium

A. cherry, banana, apple

B. apple, banana, cherry

C. banana, cherry, apple

D. apple, cherry, banana

Solution

1. Step 1: Trace enqueue order

   Elements are added in order: apple, then banana, then cherry.

2. Step 2: Understand queue order

   Queue keeps order of insertion, so printing shows apple, banana, cherry.

3. Final Answer:

   apple, banana, cherry -> Option B

4. Quick Check:

   Queue prints in enqueue order [OK]

Hint: Queue keeps insertion order; output matches enqueue sequence [OK]

Common Mistakes:

• Assuming reverse order output
• Mixing enqueue with stack behavior
• Confusing output order with dequeue

4. Identify the error in the following code snippet that tries to enqueue an element into a queue:

Queue<int> q = new Queue<int>();
q.Enqueue(5);
q.Enqueue(10);
q.Enqueue(15);
q.Enqueue();

medium

A. Cannot enqueue integer values

B. Queue is not initialized

C. Enqueue method does not exist

D. Missing argument in the last Enqueue call

Solution

1. Step 1: Check method calls

   The last call q.Enqueue(); has no argument, but Enqueue requires one.

2. Step 2: Confirm method signature

   Enqueue needs a value to add, so missing argument causes error.

3. Final Answer:

   Missing argument in the last Enqueue call -> Option D

4. Quick Check:

   Enqueue requires one argument [OK]

Hint: Enqueue always needs one argument to add [OK]

Common Mistakes:

• Calling Enqueue without argument
• Assuming Enqueue can be empty
• Confusing initialization errors

5. You have a queue q with elements [1, 2, 3]. You want to enqueue elements 4 and 5, but only if the queue size is less than 5. Which code snippet correctly performs this conditional enqueue operation in C#?

hard

A. if (q.Count == 5) { q.Enqueue(4); q.Enqueue(5); }

B. if (q.Count <= 5) { q.Enqueue(4); q.Enqueue(5); }

C. if (q.Count < 5) { q.Enqueue(4); q.Enqueue(5); }

D. if (q.Count > 5) { q.Enqueue(4); q.Enqueue(5); }

Solution

1. Step 1: Understand queue size condition

   We want to add only if size is less than 5, so condition is q.Count < 5.

2. Step 2: Apply enqueue calls inside condition

   If condition is true, enqueue 4 and 5 to add them safely.

3. Final Answer:

   if (q.Count < 5) { q.Enqueue(4); q.Enqueue(5); } -> Option C

4. Quick Check:

   Use < 5 to check size before enqueue [OK]

Hint: Check queue size with Count before enqueueing [OK]

Common Mistakes:

• Using <= instead of < causing overflow
• Using > or == which are wrong conditions
• Not checking size before enqueue