Double Ended Queue Deque in DSA C++ - Build from Scratch

Double Ended Queue (Deque) Operations

📖 Scenario: You are managing a line of customers at a theme park ride. Sometimes customers join the line at the front (VIPs), and sometimes at the back (regular visitors). You need to keep track of the order of customers efficiently.

🎯 Goal: Build a program that uses a deque to add customers to the front and back of the line, remove customers from both ends, and then display the final order of customers waiting.

📋 What You'll Learn

Use the C++ Standard Library deque to store customer names as strings.

Add customers to the front and back of the deque.

Remove customers from the front and back of the deque.

Print the final state of the deque showing the order of customers.

💡 Why This Matters

🌍 Real World

Deques are useful in real life for managing lines where people can join or leave from both ends, like theme park queues or task scheduling.

💼 Career

Understanding deques helps in software development roles that require efficient data handling, such as system programming, game development, and real-time applications.

1

Create the initial deque with customers

Create a deque<string> called line and initialize it with these exact customers in order: "Alice", "Bob", "Charlie".

DSA C++

#include 
#include 
#include 

int main() {
    // Create deque called line with "Alice", "Bob", "Charlie"
    // Your code here

    return 0;
}

Hint

Use std::deque<std::string> line = {"Alice", "Bob", "Charlie"}; to create and initialize the deque.

2

Add customers to the front and back

Add "Diana" to the front of line using push_front and add "Ethan" to the back of line using push_back.

DSA C++

#include 
#include 
#include 

int main() {
    std::deque line = {"Alice", "Bob", "Charlie"};
    // Add "Diana" to the front of line
    // Add "Ethan" to the back of line
    // Your code here

    return 0;
}

Hint

Use line.push_front("Diana") and line.push_back("Ethan") to add customers.

3

Remove customers from front and back

Remove one customer from the front of line using pop_front and one customer from the back of line using pop_back.

DSA C++

#include 
#include 
#include 

int main() {
    std::deque line = {"Alice", "Bob", "Charlie"};
    line.push_front("Diana");
    line.push_back("Ethan");
    // Remove one customer from the front
    // Remove one customer from the back
    // Your code here

    return 0;
}

Hint

Use line.pop_front() and line.pop_back() to remove customers.

4

Print the final order of customers

Use a for loop with const std::string& customer to iterate over line and print each customer followed by a space. Then print a newline.

DSA C++

#include 
#include 
#include 

int main() {
    std::deque line = {"Alice", "Bob", "Charlie"};
    line.push_front("Diana");
    line.push_back("Ethan");
    line.pop_front();
    line.pop_back();
    // Print each customer in line separated by space
    // Your code here

    return 0;
}

Hint

Use a for loop to print each customer with a space, then print a newline.

Practice

1. What is a Double Ended Queue (Deque)?

easy

A. A stack where elements are added and removed only from the top

B. A queue where elements are added only at the rear and removed from the front

C. A data structure where elements can be added or removed from both ends

D. A list where elements can be accessed randomly by index

Solution

1. Step 1: Understand the definition of Deque

   A Deque allows insertion and deletion at both front and rear ends, unlike a queue or stack.

2. Step 2: Compare with other data structures

   Queue allows operations at one end for insertion and the other for deletion; stack allows operations only at one end.

3. Final Answer:

   A data structure where elements can be added or removed from both ends -> Option C

4. Quick Check:

   Deque = double-ended queue allowing both ends operations [OK]

Hint: Deque means double-ended queue, both ends usable [OK]

Common Mistakes:

• Confusing deque with stack or queue
• Thinking deque allows random access like arrays
• Assuming deque only works like a queue

2. Which of the following is the correct syntax to declare a deque of integers in C++?

easy

A. deque<int> d = new deque();

B. deque<int> d;

C. deque<int> d[];

D. deque d<int>;

Solution

1. Step 1: Recall C++ STL deque declaration syntax

   The correct syntax uses template syntax: deque<int> d; declares a deque of integers named d.

2. Step 2: Check other options for syntax errors

   'deque<int> d = new deque();' uses invalid 'new' syntax for STL containers, 'deque d<int>;' has wrong template placement, 'deque<int> d[];' declares an array of deques incorrectly.

3. Final Answer:

   deque<int> d; -> Option B

4. Quick Check:

   Correct C++ deque declaration = deque<int> d; [OK]

Hint: Use template syntax: deque<Type> name; [OK]

Common Mistakes:

• Placing template brackets incorrectly
• Using Java or other language syntax
• Declaring arrays of deques without proper syntax

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

#include <iostream>
#include <deque>
using namespace std;
int main() {
  deque<int> d;
  d.push_back(10);
  d.push_front(20);
  d.push_back(30);
  d.pop_front();
  for (auto x : d) cout << x << " ";
  return 0;
}

medium

A. 10 30

B. 30 10

C. 20 10 30

D. 10 20 30

Solution

1. Step 1: Trace deque operations step-by-step

   Initially empty deque. push_back(10) -> [10]. push_front(20) -> [20, 10]. push_back(30) -> [20, 10, 30]. pop_front() removes 20 -> [10, 30].

2. Step 2: Print elements in deque

   Loop prints elements in order: 10 30

3. Final Answer:

   10 30 -> Option A

4. Quick Check:

   After pop_front, deque = 10 30 [OK]

Hint: Track push_front/back and pop_front carefully [OK]

Common Mistakes:

• Forgetting pop_front removes front element
• Mixing order of push_front and push_back
• Assuming pop_front removes from back

4. Identify the error in the following C++ code using deque:

#include <deque>
using namespace std;
int main() {
  deque<int> d;
  d.push_front(5);
  d.pop_back();
  d.pop_back();
  return 0;
}

medium

A. deque<int> d; is invalid declaration

B. push_front() is not a valid deque method

C. Missing #include <iostream> for cout

D. Calling pop_back() on empty deque causes undefined behavior

Solution

1. Step 1: Analyze deque operations

   After push_front(5), deque has one element [5]. First pop_back() removes 5, deque becomes empty []. Second pop_back() is called on empty deque.

2. Step 2: Understand behavior of pop_back on empty deque

   Calling pop_back() on empty deque causes undefined behavior or runtime error.

3. Final Answer:

   Calling pop_back() on empty deque causes undefined behavior -> Option D

4. Quick Check:

   pop_back on empty deque = error [OK]

Hint: Never pop from empty deque to avoid errors [OK]

Common Mistakes:

• Assuming pop_back on empty deque is safe
• Confusing push_front with invalid method
• Ignoring missing iostream is irrelevant here

5. You want to implement a sliding window maximum using a deque in C++. Which deque operation is essential to maintain the window elements in decreasing order?

hard

A. Remove elements from the back while they are smaller than the new element

B. Remove elements from the front while they are smaller than the new element

C. Add new elements only at the front

D. Add new elements only at the back without removal

Solution

1. Step 1: Understand sliding window maximum logic

   To keep maximum at front, remove all smaller elements from back before adding new element at back.

2. Step 2: Identify correct deque operation

   Removing smaller elements from back maintains decreasing order, ensuring front is max.

3. Final Answer:

   Remove elements from the back while they are smaller than the new element -> Option A

4. Quick Check:

   Sliding window max uses back removals for order [OK]

Hint: Remove smaller from back to keep max at front [OK]

Common Mistakes:

• Removing from front instead of back
• Adding elements without removing smaller ones
• Adding only at front breaks order