📚 Stack – Ngăn xếp LIFO

#include 
using namespace std;

bool isMatching(char open, char close) {
    return (open == '(' && close == ')')
        || (open == '[' && close == ']')
        || (open == '{' && close == '}');
}

int main() {
    string s;
    cin >> s;

    stack st;

    for (char c : s) {
        if (c == '(' || c == '[' || c == '{') {
            st.push(c);
        } else {
            if (st.empty() || !isMatching(st.top(), c)) {
                cout << "NO";
                return 0;
            }

            st.pop();
        }
    }

    cout << (st.empty() ? "YES" : "NO");
}

#include 
using namespace std;

bool isNumber(const string& token) {
    if (token.empty()) return false;

    int position = 0;

    if (token[0] == '-' && token.size() > 1) {
        position = 1;
    }

    for (; position < (int)token.size(); position++) {
        if (!isdigit(token[position])) return false;
    }

    return true;
}

int main() {
    string expression;
    getline(cin, expression);

    stringstream stream(expression);
    string token;
    stack st;

    while (stream >> token) {
        if (isNumber(token)) {
            st.push(stoll(token));
            continue;
        }

        long long right = st.top();
        st.pop();

        long long left = st.top();
        st.pop();

        if (token == "+") st.push(left + right);
        if (token == "-") st.push(left - right);
        if (token == "*") st.push(left * right);
        if (token == "/") st.push(left / right);
    }

    cout << st.top();
}

#include 
using namespace std;

int main() {
    int n;
    cin >> n;

    vector a(n);
    vector answer(n, -1);

    for (long long &x : a) cin >> x;

    stack st;

    for (int i = 0; i < n; i++) {
        while (!st.empty() && a[st.top()] < a[i]) {
            answer[st.top()] = a[i];
            st.pop();
        }

        st.push(i);
    }

    for (long long x : answer) {
        cout << x << ' ';
    }
}

def valid_brackets(text):
    stack = []
    matching = {
        ')': '(',
        ']': '[',
        '}': '{'
    }

    for character in text:
        if character in '([{':
            stack.append(character)
        else:
            if not stack or stack[-1] != matching[character]:
                return False

            stack.pop()

    return not stack