Prefix to Postfix Conversion

Algo prefix_2_postfix (prefix)
{    // input- valid prefix expression
    //output- equivalent postfix expression
1.    Createstack (stack).
2.    i=sizeof(prefix)-1.
3.    loop(i>=0)
    {
    a.if prefix[i] is an operator
        {
        operand2=popstack().
        operand1=popstack().
        temp= concatenate  operand1 + operand2  + prefix[i].
         pushstack (temp).
            }
    b.else if prefix[i] is an operand
        Then push prefix[i] into stack.
    c.Decrement  i  with 1.
      }
4.    postfix= popstack().
5.    Return postfix. 
}// END OF ALGO.

/* Program for Prefix to Postfix Conversion */
#include <iostream>
#include <string>
#define sizes 100
using namespace std;
class stack
{
    string item[sizes];
    int top;
    public:
        stack()
        {
            top=-1;
        }
        void push(string str)
        {
            if(top==sizes-1)
            {
                cout<<"stack overflow!!";
                return;
            }
            top++;
            item[top]=str;
        }
        string pop()
        {
            if(top==-1)
            {
                cout<<"stack underflow!!";
                return "abc";
            }
            string temp;
            temp=item[top];
            top--;
            return temp;
        }
};
int main(int argc, char** argv)
 {
     int i;
    string prefix;
    stack st;
    cout<<"Enter valid prefix expression: ";
    cin>>prefix;
    for(i=prefix.size()-1;i>=0;i--)
    {
        if(prefix[i]=='+' || prefix[i]=='-' || prefix[i]=='*' || prefix[i]=='/' || prefix[i]=='^')
        {
            string op1,op2,temp;
            op1=st.pop();
            op2=st.pop();
            temp=op1+op2+prefix[i];
            st.push(temp);
        }
        else
        {
            string flag;
            flag=flag+prefix[i];
            st.push(flag);
        }
    }
    cout<<"The equivalent postfix expression: "<<st.pop();
    return 0;
}