isOperator function
Returns true if s represents an arithmetic operator, false otherwise.
This function checks the input string against a set of predefined
arithmetic operators. If the input string matches any of the operators,
the function returns true. Otherwise, it returns false.
Implementation
bool isOperator(String s) {
return s == '+' || s == '-' || s == '*' || s == '/' || s == '×' || s == '÷';
}