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

using namespace std;

int main(){

    //stack declaration for integers
    stack<int> varStack;

    string usrInput;

    /* 
     * Separate string to concatenate multi-digit int 
     * Used in default case for first appearance of an int in user input
     */
    string multiDigitVar;

    //integers for evaluation
    int var1, var2, answer;

    //Accept user input as entire line 
    cout << "Please enter your equation in Reverse Polish Notation: ";
    getline(cin, usrInput);
    cout << endl;

    //FOR loop continuing length of user input
    for(int i = 0; i < usrInput.length(); i++)
    {
        //necessary integer to convert string to int
        int convertedVar;

        switch(usrInput[i]){
            case '+': //addition
                var2 = varStack.top();
                varStack.pop();

                var1 = varStack.top();
                varStack.pop();

                answer = var1 + var2;
                varStack.push(answer);

                break;
            case '-': //subtraction
                var2 = varStack.top();
                varStack.pop();

                var1 = varStack.top();
                varStack.pop();

                answer = var1 - var2;
                varStack.push(answer);

                break;
            case '*': //multiplication
                var2 = varStack.top();
                varStack.pop();

                var1 = varStack.top();
                varStack.pop();

                answer = var1 * var2;
                varStack.push(answer);
                break;
            case '/': //division
                var2 = varStack.top();
                varStack.pop();

                var1 = varStack.top();
                varStack.pop();

                answer = var1 / var2;
                varStack.push(answer);
                break;
            case ' ':
                //white space in user Input. Do nothing.
                break;
            default:
                if(usrInput[i+1] != ' ') { //tests for multi-digit number in response
                    int j = i;
                    while (usrInput[j] != ' ') //while loop adds digits until whitespace appears in index + 1
                    {
                        multiDigitVar += usrInput[j]; //form the number by applying each digit

                        j++; //traverse multi-digit number
                        i++; //update index counter
                    }

                    convertedVar = stoi(multiDigitVar);
                    varStack.push(convertedVar);
                    break;
                }
                //single-digit conversion and insertion
                convertedVar = usrInput[i] - '0'; //convert character to int using special technique
                varStack.push(convertedVar);
                break;
        }

        //reset multi-digit string for next appearance of multi-digit int
        multiDigitVar = ' ';
    }

    //output result
    cout << "The result of your equation is: ";
    cout << varStack.top();
    cout << endl;
}