string convertPostfix(string infix) {
	string postFix;
	stack<char> numStack;

	for (int index = 0; index < infix.length(); index++) {
		if (isdigit(infix.at(index))) {
			postFix += (infix.at(index));
		}

		else if (infix.at(index) == '(') {
			numStack.push(infix.at(index));
		}

		else if (infix.at(index) == '+' || infix.at(index) == '-' || infix.at(index) == '*' || infix.at(index) == '/') {
			if (!numStack.empty()) {
				while (!numStack.empty() && (hasHigherPrecedence(numStack.top(), infix.at(index)))) {
					postFix += numStack.top();
					numStack.pop();
				}
				numStack.push(infix.at(index));
			}
			else if (numStack.empty())
				numStack.push(infix.at(index));
		}
		else if (infix.at(index) == ')') {
			while (numStack.empty() == false && numStack.top() != '(') {
				postFix += numStack.top();
				numStack.pop();
			}
			numStack.pop();
		}
	}

	while (!numStack.empty()) {
		if (numStack.top() != '(' && numStack.top() != ')') {
			postFix += numStack.top();
			numStack.pop();
		}
		else {
			numStack.pop();
		}
	}
	return postFix;
}