c++前缀求值补全代码的问题

#include <iostream>
#include <vector>
#include <sstream>

int evaluatePrefixExpression(std::vector<std::string>& tokens, int& index) {
    std::string token = tokens[index];

    if (isOperator(token)) {
        int operand1 = evaluatePrefixExpression(tokens, ++index);
        int operand2 = evaluatePrefixExpression(tokens, ++index);

        if (token == "+")
            return operand1 + operand2;
        else if (token == "-")
            return operand1 - operand2;
        else if (token == "*")
            return operand1 * operand2;
        else if (token == "/")
            return operand1 / operand2;
    }
    else {
        // Convert the token to an integer
        std::stringstream ss(token);
        int operand;
        ss >> operand;
        return operand;
    }

    return 0; // Default return value (should not reach here if the input is valid)
}

int evaluatePrefixExpression(std::vector<std::string>& tokens) {
    int index = 0;
    return evaluatePrefixExpression(tokens, index);
}

int main() {
    std::vector<std::string> tokens = { "*", "+", "2", "3", "4" };
    int result = evaluatePrefixExpression(tokens);
    std::cout << "Result: " << result << std::endl;
    return 0;
}