栈计算逆波兰表达式

#include<stack>
#include<math.h>
#include<iostream>
#include<string.h>

using namespace std;

const char* s1 = "8 4 + 6 2 * -";
const char* s2 = "2 3 5 + * 7 1 / + 4 -";

float calculation(float i, char j, float k)
{
    if ('!' == j)
    {
        float factorial = 1;
        for (int a = 1; a <= i; ++a)
        {
            factorial *= a;
        }
        return factorial;
    }
    else if ('+' == j)
    {
        return k + i;
    }
    else if ('-' == j)
    {
        return k - i;
    }
    else if ('*' == j)
    {
        return k * i;
    }
    else if ('/' == j)
    {
        return k / i;
    }
    else if ('^' == j)
    {
        return pow(k, i);
    }
}

float calculationStr(const char* S)
{
    stack<float> numStack;
    for (int i = 0; i < strlen(S); ++i)
    {
        if(isalnum(S[i]))
        {
            string str = "";
             while(isalnum(S[i]))
             {    
                 str += S[i++];    
            } 
            --i;
            numStack.push(atof(str.c_str()));
        }
        else if(' ' != S[i] && '\0' != S[i])
        {
            if('!' == S[i])
            {
                float num1 = numStack.top();
                numStack.pop();
                numStack.push(calculation(num1, S[i], 0));
            }
            else
            {
                float num1 = numStack.top();
                numStack.pop();
                float num2 = numStack.top();
                numStack.pop();
                numStack.push(calculation(num1, S[i], num2));
            }
        }
    }
    return numStack.top();
} 

int main()
{
    cout<<calculationStr(s1)<<endl;
    cout<<calculationStr(s2)<<endl;
}