Infix to prefix & postfix

#include<bits/stdc++.h>
#include <conio.h>
using namespace std;

bool isOperator(char c)
{
	return (!isalpha(c) && !isdigit(c));
}

int getPriority(char C)
{
	if (C == '-' || C == '+')
		return 1;
	else if (C == '*' || C == '/')
		return 2;
	else if (C == '^')
		return 3;
	return 0;
} 

string infixToPostfix(string infix)
{
	infix = '(' + infix + ')';
	int l = infix.size();
	stack<char> char_stack;
	string output;
		for (int i = 0; i < l; i++) {
		if (isalpha(infix[i]) || isdigit(infix[i]))
			output += infix[i];
		else if (infix[i] == '(')
			char_stack.push('(');
		else if (infix[i] == ')') {
			while (char_stack.top() != '(') {
				output += char_stack.top();
				char_stack.pop();
			}
			char_stack.pop();
		} else {
			if (isOperator(char_stack.top())) {
				while (getPriority(infix[i]) <= getPriority(char_stack.top())) {
				output += char_stack.top();
				char_stack.pop();
				}
				char_stack.push(infix[i]);
			}
		}
	}
	return output;
}

string infixToPrefix(string infix)
{
	int l = infix.size();
	reverse(infix.begin(), infix.end());
	for (int i = 0; i < l; i++) {
		if (infix[i] == '(') {
			infix[i] = ')';
			i++;
		} else if (infix[i] == ')') {
			infix[i] = '(';
			i++;
		}
	}
	string prefix = infixToPostfix(infix);
	reverse(prefix.begin(), prefix.end());
	return prefix;
}

int main()
{
	char ulang;
	string infix;
	do {
		system("cls");
		cout << "Masukan infix: "; cin >> infix;
		cout << "Prefix: "<<infixToPrefix(infix)<<endl;
		cout << "Postfix: "<< infixToPostfix(infix)<<endl;
		cout << "Ingin melanjutkan lagi? [y/n]"; cin >> ulang;
	} while(ulang == 'y');
	cout<<"Tidak mengulang dan terima kasih"<<endl;
	getch();
}