STL Implementation

#include <bits/stdc++.h> // Includes standard libraries, including <stack> and <iostream>
using namespace std;     // Allows using std:: functions directly (e.g., cout, stack)

int main() {
    // Creation of stack
    stack<int> s; // Declares a stack named 's' that will store integer values

    // Push operation
    s.push(10); // Adds 10 to the top of the stack
    s.push(61); // Adds 61 to the top of the stack (now 61 is the top element)

    // Size operation
    cout << "Current size of stack : " << s.size() << endl; // Prints the number of elements in the stack

    // Peek operation
    cout << "Top element : " << s.top() << endl; // Prints the top element without removing it

    // Pop operation
    s.pop(); // Removes the top element (61 is removed, 10 becomes the new top)

    cout << "Current size of stack : " << s.size() << endl; // Prints the updated size
    cout << "Top element : " << s.top() << endl;             // Prints the new top element (which is 10)

    // Check if stack is empty
    if(s.empty()) { // Checks if the stack has no elements
        cout << "Stack is empty!" << endl;
    } else {
        cout << "Stack is not empty!" << endl;
    }

    s.pop(); // Removes the last element (10 is removed, stack becomes empty)

    // Check if stack is empty again
    if(s.empty()) {
        cout << "Stack is empty!" << endl; // This condition will now be true
    } else {
        cout << "Stack is not empty!" << endl;
    }

    cout << "Current size of stack : " << s.size() << endl; // Prints the final size (which is 0)
    return 0; // Indicates successful program execution
}

Here are quick revision notes for the provided C++ stack code, formatted in Markdown for easy copying:

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Key Operations & Concepts

• stack<int> s;
  • Comment: Creates a stack s that stores int type elements. Stacks follow a LIFO (Last-In, First-Out) principle.
• s.push(element);
  • Comment: Adds element to the top of the stack.
  • Example: s.push(10); s.push(61); (Stack: [10, 61] where 61 is on top)
• s.size();
  • Comment: Returns the number of elements currently in the stack.
  • Example: After s.push(10); s.push(61);, s.size() is 2.
• s.top();
  • Comment: Returns a reference to the top element of the stack. Does not remove the element. Calling top() on an empty stack leads to undefined behavior.
  • Example: After s.push(10); s.push(61);, s.top() is 61.
• s.pop();
  • Comment: Removes the top element from the stack. Does not return the removed element. Calling pop() on an empty stack leads to undefined behavior.
  • Example: After s.push(10); s.push(61); s.pop();, stack becomes [10].
• s.empty();
  • Comment: Returns true if the stack contains no elements, false otherwise. Useful to prevent top() or pop() on an empty stack.
  • Example: If stack is [], s.empty() is true.

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Execution Flow & Output Trace

• Initial: s is empty.
• s.push(10); s.push(61);
  • Stack: [10, 61] (61 is top)
• cout << s.size();
  • Output: Current size of stack : 2
• cout << s.top();
  • Output: Top element : 61
• s.pop();
  • Stack: [10] (10 is top)
• cout << s.size();
  • Output: Current size of stack : 1
• cout << s.top();
  • Output: Top element : 10
• if(s.empty()) { ... } else { ... }
  • s is not empty, so output: Stack is not empty!
• s.pop();
  • Stack: [] (empty)
• if(s.empty()) { ... } else { ... }
  • s is empty, so output: Stack is empty!
• cout << s.size();
  • Output: Current size of stack : 0

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