C++ std::stack: Last-In First-Out (LIFO) Container Adapter & Real Use Cases

std::stack is a Container Adapter that provides a strict LIFO (Last-In, First-Out) interface on top of an underlying container (by default, std::deque).

Classic Interview Problem: Balanced Parentheses Checker

#include <iostream>
#include <stack>
#include <string>

bool isBalanced(const std::string& expr) {
    std::stack<char> s;
    for (char ch : expr) {
        if (ch == '(' || ch == '{' || ch == '[') {
            s.push(ch);
        } else if (ch == ')' || ch == '}' || ch == ']') {
            if (s.empty()) return false;
            char top = s.top();
            s.pop();
            if ((ch == ')' && top != '(') ||
                (ch == '}' && top != '{') ||
                (ch == ']' && top != '[')) {
                return false;
            }
        }
    }
    return s.empty();
}

int main() {
    std::cout << std::boolalpha;
    std::cout << "{[()]} is balanced: " << isBalanced("{[()]}") << "\n"; // true
    std::cout << "{[(])} is balanced: " << isBalanced("{[(])}") << "\n"; // false
    return 0;
}