3. Commands

#include <iostream>
#include <string>
#include <stack>
#include <queue>
using namespace std;

int sum(stack<int> & numbers)
{
    int result = 0;
    int num1 = numbers.top();
    numbers.pop();
    int num2 = numbers.top();
    numbers.pop();

    result = num1 + num2;
    return result;
}

int subtract(stack<int>& numbers)
{
    int result = 0;
    int num1 = numbers.top();
    numbers.pop();
    int num2 = numbers.top();
    numbers.pop();

    result = num1 - num2;
    return result;
}

void concat(stack<int>& numbers)
{
    string num1 = to_string(numbers.top());
    numbers.pop();
    string num2 = to_string(numbers.top());
    numbers.pop();

    string result = num2 + num1;
    int number = stoi(result);
    numbers.push(number);
}

void discard(stack<int>& numbers)
{
    numbers.pop();
}

void transformToQueue(stack<int>& numbers, stack<int> & temp, queue<int> & tempQueue)
{
    while (!numbers.empty())
    {
        temp.push(numbers.top());
        numbers.pop();
    }

    while (!temp.empty())
    {
        tempQueue.push(temp.top());
        temp.pop();
    }
}

int main()
{
    stack<int> sequence;
    string input;
    cin >> input;
    while (input != "end")
    {
        if (isdigit(input[0]) || input[0] == '-')
        {
            int number = stoi(input);
            sequence.push(number);
        }
        else if (input == "sum")
        {
            int number = sum(sequence);
            sequence.push(number);
        }
        else if (input == "subtract")
        {
            int number = subtract(sequence);
            sequence.push(number);
        }
        else if (input == "concat")
        {
            concat(sequence);
        }
        else if (input == "discard")
        {
            discard(sequence);
        }

        cin >> input;
    }

    queue<int> output;
    stack<int> temp;
    transformToQueue(sequence, temp, output);

    while (!output.empty())
    {
        cout << output.front() << endl;
        output.pop();
    }

    return 0;
}