Stack and Queue in C++

Queue

Operations: push(), pop(), empty(), front(), back()

Stack

Operations: push(), pop(), empty(), top()

Key Difference:

Queue → FIFO (First In First Out)
Stack → LIFO (Last In First Out)

Queue & Stack Initialization

Queue Initialization & Access

    queue<int> myQue; // Initialize empty
    for(int i = 1; i <= 3; i++) myQue.push(i); // Push in loop

❌ queue<int> myQue = {1, 2, 3}; ⭐ not allowed
✅ queue<int> myQue({1, 2, 3}); ⭐ // using constructor

Stack Initialization & Access

    stack<int> mySt; // Initialize empty
    for(int i = 1; i <= 3; i++) mySt.push(i); // Push in loop

❌ stack<int> mySt = {1, 2, 3}; → not allowed
✅ stack<int> mySt({1, 2, 3}); → works using constructor (rarely used)

Queue vs Stack Operations

Queue (FIFO)

✅ myQue.size() → number of elements
❌ myQue[0] → direct indexing not allowed
✅ myQue.front() → access first element
✅ myQue.back() → access last element
✅ myQue.push(x) → add element at back
✅ myQue.pop() → remove element from front

Stack (LIFO)

✅ mySt.size() → number of elements
❌ mySt[0] → direct indexing not allowed
✅ mySt.top() → access top element
✅ mySt.push(x) → add element at top
✅ mySt.pop() → remove top element

Stack #include <stack>

std::stack<int> s;

// Push elements onto the stack
s.push(1);
s.push(2);
s.push(3);

// Display the top element
std::cout << "Top element: " << s.top() << std::endl;

// Pop the top element
s.pop();

// Display the new top element
std::cout << "New top element: " << s.top() << std::endl;

// Check if the stack is empty
if (s.empty()) {
  std::cout << "Stack is empty" << std::endl;
} else {
  std::cout << "Stack size: " << s.size() << std::endl;
}

Queue #include <queue>

std::queue<int> q;

// Push elements into the queue
q.push(1);
q.push(2);
q.push(3);

// Display the front element
std::cout << "Front element: " << q.front() << std::endl;

// Pop the front element
q.pop();

// Display the new front element
std::cout << "New front element: " << q.front() << std::endl;

// Check if the queue is empty
if (q.empty()) {
  std::cout << "Queue is empty" << std::endl;
} else {
  std::cout << "Queue size: " << q.size() << std::endl;
}

Additional Notes

Both std::stack and std::queue are container adapters that use other underlying containers (like std::deque or std::list) to store elements. -By default, std::stack uses std::deque, and std::queue also uses std::deque. You can change the underlying container if needed.