C++ Program to Implement Stack in STL

Stack is a linear data structure that follows the Last In First Out (LIFO) principle. In this article, we will learn how to use the stack container from the Standard Template Library (STL) in C++.

What is Stack?

A stack is a container that stores data in a way that the last inserted data will be the first to be removed. This is same as a stack of plates, where the last plate placed on the top is the first one to be removed. The STL library of C++ provides a pre-defined stack data structure which comes with all the operations available for a stack.

For example, the below section shows how data are inserted and removed from a stack:

// Declare a stack
stack<int> s;

// Add Data
s.push(10);
s.push(20);

s.pop(); // This operation will remove 20 from stack

Using stack Container in STL

The stack container is defined in the <stack> header of STL. It provides a LIFO data structure with basic operations such as push, pop, top, and size. Below are some points about this container:

• Header: <stack>
• Syntax:

  stack<datatype> stack_name;

• Common functions:
• push() - Function used to insert an element at the top of the stack.
• pop() - Function used to remove an element from the top of the stack.
• top() - Function used to access the top element of the stack.
• empty() - Function used to check if the stack is empty.
• size() - Function used to return the size of the stack.

Steps to Implement Stack in C++ STL

Following are steps/algorithm to use stack using C++ STL:

• Include the <stack> header file.
• Declare a stack with desired data type.
• Use push() to insert elements.
• Use pop() to remove elements from top.
• Access elements using top().
• Use empty() and size() to check status.

C++ Program to Implement Stack using STL

The below code is the implementation of the above algorithm in C++ language.

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

int main() {
    stack<int> s;

    // Insert elements
    s.push(10);
    s.push(20);
    s.push(30);

    cout << "Top Element: " << s.top() << endl;

    // Remove one element
    s.pop();

    cout << "After one pop operation:" << endl;
    cout << "Top Element: " << s.top() << endl;
    cout << "Stack Size: " << s.size() << endl;

    // Check if empty
    if (s.empty()) {
        cout << "Stack is empty" << endl;
    } else {
        cout << "Stack is not empty" << endl;
    }

    return 0;
}

The output of above code will be

Top Element: 30
After one pop operation:
Top Element: 20
Stack Size: 2
Stack is not empty

Time and Space Complexity

Time Complexity:

• push(), pop(), top(): O(1) for each operation.

Space Complexity: O(n), where n is the number of elements in the stack.