1406-stack.cpp

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

using namespace std;

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

    stack<char> left, right;
    for (char c : statement)
        left.push(c);

    int M;
    cin >> M;
    cin.ignore();

    for (int i = 0; i < M; ++i) {
        string command;
        getline(cin, command);

        switch (command[0]) {
            case 'L':
                if (!left.empty()) {
                    right.push(left.top());
                    left.pop();
                }
                break;
            case 'D':
                if (!right.empty()) {
                    left.push(right.top());
                    right.pop();
                }
                break;
            case 'B':
                if (!left.empty()) {
                    left.pop();
                }
                break;
            case 'P':
                left.push(command[2]);
                break;
        }
    }

    while (!left.empty()) {
        right.push(left.top());
        left.pop();
    }

    while (!right.empty()) {
        cout << right.top();
        right.pop();
    }

    return 0;
}