Learning data structures in C++ is one of the most important steps for anyone looking to become a skilled programmer or software engineer. Data structures provide systematic ways to store and organize data, making programs faster and more efficient. With C++, you can implement data structures in two powerful ways: by building them from scratch or by using the Standard Template Library (STL), which offers ready-to-use implementations. From arrays, linked lists, and stacks to advanced trees, graphs, and hash maps, this guide will introduce the essential C++ data structures with examples to help you strengthen your problem-solving skills, prepare for coding interviews, and apply them in real-world projects.

What is STL in C++ :

The Standard Template Library (STL) in C++ is a powerful set of template classes that provide common data structures and algorithms. It helps programmers save time by reusing well-tested components instead of writing everything from scratch. STL mainly consists of containers, algorithms, iterators, and functors.

Data Structures in C++

A data structure is a way of organizing and storing data so it can be used efficiently.
In C++, common data structures include arrays, linked lists, stacks, queues, trees, graphs, and hash tables.

With STL, many of these are already implemented, such as:

Vector (dynamic array)
List (doubly linked list)
Stack and Queue
Set and Map (associative containers)

These tools allow developers to handle data effectively and apply algorithms like searching, sorting, and traversing with ease.

Here’s a short comparison table for the mentioned STL containers in C++:

Container	Type	Key Features	Use Case
Vector	Dynamic Array	Stores elements in contiguous memory Fast random access (O(1)) Insertion/deletion at end is fast, but slow in the middle	When frequent random access is needed
List	Doubly Linked List	Each element has pointers to prev & next No random access Fast insertion/deletion anywhere (O(1) with iterator)	When frequent insertions/deletions are needed
Stack	Container Adapter	LIFO (Last In, First Out) push(), pop(), top()	Undo operations, expression evaluation
Queue	Container Adapter	FIFO (First In, First Out) push(), pop(), front(), back()	Task scheduling, order processing
Set	Associative Container	Stores unique, sorted elements Based on balanced BST Fast search (O(log n))	When unique, sorted data is needed
Map	Associative Container	Stores key–value pairs, sorted by key No duplicate keys Fast search/retrieval (O(log n))	Dictionaries, key-value

C++ Vectors: Basic to Advanced:

1. What is a Vector? (Basics)

A vector is a dynamic array in C++ provided by the Standard Template Library (STL).
Unlike regular arrays, vectors can resize automatically when you add or remove elements.
Elements are stored contiguously in memory → random access is fast (O(1) time).

Declaration:

vector<int> v1; // empty vector
vector<int>v2(5); // vector of size 5, default values 0
vector <int>v3(5, 10); // vector of size 5, all values 10
vector<int> v4 = {1, 2, 3}; // initializer list

Insert elements:

v1.push_back(10); // add at end
v1.emplace_back(20); // faster, constructs in place

Access elements:

cout << v1[0]; // no bounds checking
cout << v1.at(0); // throws error if index out of bounds
cout << v1.front(); // first element
cout << v1.back(); // last element

Remove elements:

v1.pop_back(); // removes last element
v1.clear(); // remove all elements

Size and capacity:

v1.size(); // number of elements
v1.capacity(); // memory allocated, may be larger than size
v1.empty(); // true if vector has no elements

Loops:

for (int i = 0; i < v1.size(); i++)
cout << v1[i] << ” “;

Range-based for loop

for (int x : v1)
cout << x << ” “;

Using iterators:

for (auto it = v1.begin(); it != v1.end(); ++it)
cout << *it << ” “;

Common STL Functions (DSA useful):

Sorting:

Searching:

Max/Min:

Advanced Vector Usage (DSA Patterns):

2D Vectors Like 2D Array :

A vector of vectors → useful for matrices, graphs, DP tables

vector<vector<int>> matrix(3, vector(4, 0)); // 3×4 matrix of 0s
matrix[1][2] = 5; // set element

Dynamic Resizing & Memory

Vectors automatically resize when push_back exceeds capacity.
v.reserve(n) → pre-allocate memory to reduce resizing overhead.
v.shrink_to_fit() → reduce capacity to current size.

Stack / Queue using vector:

vector<int> stack;
stack.push_back(10); // push
stack.pop_back(); // pop
int top = stack.back(); // top

Vector of Pairs:

vector<pair<int,int>> vp;
vp.push_back({1,2});
vp.emplace_back(3,4);

for(auto &p : vp)
cout << p.first << ” ” << p.second << endl;

Vector of Custom Objects:

struct Point { int x, y; };
vector <point> points;
points.push_back({1,2});
points.emplace_back(Point{3,4});

Common Mistakes in DSA:

Accessing v[i] when v is empty use v.at(i) for safety.
Forgetting that vector indices start at 0.
Using push_back in a loop over empty vector incorrectly (common for beginners).
Using size() in loop as int → better to use size_t to avoid warnings:

TASKs:

Write a program to input n integers into a vector<int> and print them. //easy no solution
Write a program to find the maximum and minimum element in a vector. //easy no solution
Write a program to reverse a vector without using the built-in reverse() function. //easy no solution
Write a program to search for an element in a vector (linear search).
Write a program to remove duplicate elements from a vector.
Write a program to sort a vector in ascending order using sort(). //easy no solution
Write a program to add two vectors element-wise and store the result in a new vector.
Write a program to rotate a vector k times to the right.