Evaluation of prefix and postfix notations in CPP

cpp-prefix-postfix

#include <iostream>
#include <stack>
#include <cmath>

using namespace std;

bool isOperator(char x)
{
    if(x == '+' || x == '-' || x == '*' || x == '/' || x == '^')
    {
        return true;
    }
    return false;
}

int evaluateOperator(int x, int y, char o)
{
    if(o == '+')
    {
        return (x+y);
    }
    else if(o == '-')
    {
        return (x-y);
    }
    else if(o == '/')
    {
        return (x/y);
    }
    else if(o == '*')
    {
        return (x*y);
    }
    else if(o == '^')
    {
        return pow(x,y);
    }

    return -1;
}

int main()
{

     /*
    PREFIX
    */

        stack<int> myStack;

       string x = "++2*32/62";
  //     string x = "22+";

        x = '(' + x;

        int len = x.length() -1;
       // cout<<len;
        bool flag = true;

        for(int i = len; x[i] != '('; i--)
        {
            if(x[i] >= '0' && x[i]<= '9')
            {
                myStack.push(x[i] - 48);
            }
            else if(myStack.empty())
            {
                flag = false;
            }
            else if(isOperator(x[i]))
            {
                int a = myStack.top();
                myStack.pop();
                int b = myStack.top();
                myStack.pop();

                int res = evaluateOperator(a,b,x[i]);

                myStack.push(res);

            }


        }


        if(flag)
        {
            cout<<myStack.top()<<endl;
        }
        else cout<<"Invalid"<<endl;

    /*
    POSTFIX
    */

    {
        stack<int> myStack;

        string x = "12+3*6+23+/";

        int len = x.length();

        x = x + ")";

        for(int i =0; i<len && x[i] != ')'; i++)
        {

            if(x[i]>='0' && x[i]<='9')
            {
                myStack.push((x[i]-48));
            }
            else
            {
                int b = myStack.top();
                myStack.pop();

                int a = myStack.top();
                myStack.pop();

                int res = 0;
                if(x[i] == '+')
                {
                    res = a + b;
                    myStack.push(res);
                }
                else if(x[i] == '-')
                {
                    res = a - b;
                    myStack.push(res);
                }
                else if(x[i] == '/')
                {
                    res = a / b;
                    myStack.push(res);
                }
                else if(x[i] == '*')
                {
                    res = a * b;
                    myStack.push(res);
                }
                else if(x[i] == '^')
                {
                    res = pow(a,b);
                    myStack.push(res);
                }
                else
                {
                    cout<<"INVALID CHARACTER IN STRING"<<endl;
                    break;

                }
            }

        }

        cout<<"RESULT: "<<myStack.top()<<endl;


    }

    return 0;
}