STL deque in C++ (Double-Ended Queue) 🚀

 

The STL deque (double-ended queue) is a dynamic container that allows fast insertion and deletion from both ends. It is more flexible than vector and does not require contiguous memory allocation.

✅ Key Features:
✔ Fast Insertions/Deletions at both front and back.
✔ Random Access – Supports deque[i].
✔ Resizable – Grows/shrinks dynamically.
✔ Efficient Memory Management – No need for contiguous allocation.

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1️⃣ Syntax & Declaration

#include <iostream>
#include <deque>  // Include deque library
using namespace std;

int main() {
    deque<int> dq = {10, 20, 30, 40, 50};  // Declare and initialize

    cout << "First Element: " << dq.front() << endl;
    cout << "Last Element: " << dq.back() << endl;

    return 0;
}

🔹 Output

First Element: 10
Last Element: 50

✅ Unlike vector, deque allows fast front insertions! 🚀

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2️⃣ Adding & Removing Elements

🔹 Adding Elements

deque<int> dq;
dq.push_back(10);  // Add at the back
dq.push_front(20); // Add at the front

🔹 Removing Elements

dq.pop_back();   // Removes last element
dq.pop_front();  // Removes first element

✅ Unlike vector, deque supports push_front() and pop_front().

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3️⃣ Traversing a deque

🔹 Using a Simple Loop

for (int i = 0; i < dq.size(); i++) {
    cout << dq[i] << " ";
}

🔹 Using a Range-Based Loop

for (int val : dq) {
    cout << val << " ";
}

🔹 Using Iterators

for (auto it = dq.begin(); it != dq.end(); it++) {
    cout << *it << " ";
}

✅ Supports iterators just like vector!

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4️⃣ Common deque Functions

🔹 Size & Capacity

cout << "Size: " << dq.size() << endl;

🔹 Accessing Elements

cout << dq.front();   // First element
cout << dq.back();    // Last element
cout << dq.at(2);     // Safe access

🔹 Checking if Empty

if (dq.empty()) cout << "Deque is empty";

🔹 Insert at Specific Position

dq.insert(dq.begin() + 1, 15);  // Insert 15 at index 1

🔹 Erase an Element

dq.erase(dq.begin() + 1);  // Removes element at index 1

✅ Faster than vector for front operations!

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5️⃣ Sorting a deque

#include <iostream>
#include <deque>
#include <algorithm>  // Sorting functions
using namespace std;

int main() {
    deque<int> dq = {50, 10, 40, 20, 30};

    sort(dq.begin(), dq.end());  // Sort in ascending order

    cout << "Sorted Deque: ";
    for (int val : dq) cout << val << " ";

    return 0;
}

🔹 Output

Sorted Deque: 10 20 30 40 50

✅ Uses sort() from <algorithm>.

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6️⃣ Swapping Two deques

#include <iostream>
#include <deque>
using namespace std;

int main() {
    deque<int> dq1 = {1, 2, 3};
    deque<int> dq2 = {7, 8, 9};

    dq1.swap(dq2);  // Swap contents

    cout << "After swapping:\n";
    for (int val : dq1) cout << val << " ";  // 7 8 9
    cout << endl;
    for (int val : dq2) cout << val << " ";  // 1 2 3

    return 0;
}

✅ swap() exchanges contents efficiently.

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7️⃣ Clearing a deque

dq.clear();  // Removes all elements

✅ Memory remains allocated. To free memory, use:

deque<int>().swap(dq);

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8️⃣ Summary

✔ std::deque<T> – Dynamic, double-ended queue.
✔ Supports push_back(), push_front(), pop_back(), pop_front().
✔ Fast insertions/removals at both ends (better than vector).
✔ Random access using dq[i], at(i), front(), back().
✔ Sorting via sort(dq.begin(), dq.end()).
✔ Resizable with resize(), clear(), swap().

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