Stl Stack

• push(val): Adds element to top.
• pop(): Removes top element.
• top(): Returns top element (doesn't remove).
• empty(): Checks if stack is empty.

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

cout << s.top(); // 20
s.pop();
cout << s.top(); // 10

Used for reversing data, balancing brackets, function calls (recursion stack), and evaluating mathematical expressions.