C++ Stacks

What is a Stack?

A stack is a LIFO (Last-In-First-Out) container. Elements are pushed and popped from the same end — the "top". Include <stack>.

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

int main() {
    stack<int> s;
    s.push(10);
    s.push(20);
    s.push(30);

    cout << "Top: " << s.top() << endl;   // 30
    cout << "Size: " << s.size() << endl; // 3

    while (!s.empty()) {
        cout << s.top() << " ";
        s.pop();
    }
    cout << endl;
    return 0;
}

Output:

Top: 30
Size: 3
30 20 10

Real-World Use Cases

• Undo/Redo in text editors
• Browser back button (page history)
• Function call stack in program execution
• Expression evaluation (balanced parentheses, RPN)

// Check balanced parentheses using a stack
string expr = "{[()]}";
stack<char> st;
bool balanced = true;

for (char c : expr) {
    if (c=='(' || c=='[' || c=='{') {
        st.push(c);
    } else {
        if (st.empty()) { balanced = false; break; }
        char top = st.top(); st.pop();
        if ((c==')' && top!='(') || (c==']' && top!='[') || (c=='}' && top!='{'))
            { balanced = false; break; }
    }
}
if (!st.empty()) balanced = false;
cout << (balanced ? "Balanced" : "Not balanced") << endl;  // Balanced

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

int main() {
    string input = "Hello, World!";
    stack<char> s;
    for (char c : input) s.push(c);
    string reversed = "";
    while (!s.empty()) { reversed += s.top(); s.pop(); }
    cout << reversed << endl;   // !dlroW ,olleH
    return 0;
}

Summary

• stack is LIFO: push and pop from the top.
• Methods: push, pop, top, empty, size.
• Classic use cases: undo, browser history, expression parsing.