03. Machine Commands

#include <iostream>
#include <string>
#include <vector>

using namespace std;

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

    vector<int> numbers;

    while (command != "end") {
            if (command == "sum") {
                numbers[numbers.size() - 2] += numbers[numbers.size() - 1];
                numbers.pop_back();
            }
            else if (command == "subtract") {
                numbers[numbers.size() - 1] -= numbers[numbers.size() - 2];
                numbers.erase(numbers.begin() + numbers.size() - 2);
            }
            else if (command == "concat") {
                numbers[numbers.size() - 2] = stoi(to_string(numbers[numbers.size() - 2]) + to_string(numbers[numbers.size() - 1]));
                numbers.pop_back();
            }
            else if (command == "discard") {
                numbers.pop_back();
            }
            else {
                numbers.push_back(stoi(command));
            }

        cin >> command;
    }

    for (auto& n : numbers) {
        cout << n << endl;
    }
    return 0;
}