Infix to Postfix

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

class postfix{
    stack <char>a;
public:
    int precedence(char c){
        if(c=='^'){
            return 3;
        }
        else if(c=='/' || c=='*')
        {
            return 2;
        }
        else if(c=='+' || c=='-'){
            return 1;
        }
        else{
            return -99;
        }
    }
    bool isoperator(char c){
        if(c=='*' || c=='/' || c=='^' || c== '+' || c=='-'){
            return true;
        }
        return false;
    }
    void infixtopostfix(string s){
        int i=0;
        while(s[i]!='\0'){
            if(s[i]=='('){
                a.push('(');
            }
            else if(s[i]>='0' && s[i]<='9'){
                cout<<s[i];
            }

            else if(isoperator(s[i])){
                if(a.empty()){
                    a.push(s[i]);
                }
                else if(a.top()=='('){
                    a.push(s[i]);
                }
                else if(precedence(s[i])>precedence(a.top())){
                    a.push(s[i]);
                }
                else if(precedence(s[i])<=precedence(a.top())){
                    while(!a.empty() && precedence(s[i])<=precedence(a.top())){
                        cout<<a.top();
                        a.pop();
                    }
                    a.push(s[i]);
                }
            }
            else if(s[i] ==')'){
              while(a.top()!='(' && !a.empty()){
                  cout<<a.top();
                  a.pop();
              }
              a.pop();
            }
            i++;
        }
        while(!a.empty()){
            cout<<a.top();
            a.pop();
        }
    }
};

int main(){
    postfix p;
    p.infixtopostfix("(1+2)*3^2");
}