Incorrect Validating a postfix boolean expression

bool validate(const string& postfix)
{
    /*
    A postfix expression is valid if and only if:
        1) The first two elements are operands(values), and
        2) The last element is an operator, and
        3) For every n values there are n-1 operator(s), and
    */

    char first_element = postfix[0];
    char second_element = postfix[1];
    char last_element = postfix[postfix.size() - 1];

    // Priority of 0 means that the element is an operand
    if (priority(first_element) != 0 && priority(second_element != 0))
        return false;

    if (priority(last_element) == 0)
        return false;

    int num_elements = 0;
    int num_operands = 0;
    bool not_exists = false;

    // FIX FOR NEXT TIME: Make sure that you can actually have infinite number of not statements and it will still be able to run

    for (int i = 0; i < postfix.size(); i++)
    {
        char ith_element = postfix[i];

        if (priority(ith_element) == 0 && ith_element != ' ')
            num_elements++;

        else if (priority(ith_element) == 1 || (priority(ith_element) == 2))
            num_operands++;

        else if ()
    }

    return (num_elements == (num_operands + 1));
}