补全Calculator.h文件中的Calc、add、sub、mult、divi函数，以实现计算器功能

#ifndef CALCULATOR_H
#define CALCULATOR_H

class Calculator {
public:
    static double Calc(double num1, char op, double num2);
    static double add(double num1, double num2);
    static double sub(double num1, double num2);
    static double mult(double num1, double num2);
    static double divi(double num1, double num2);
};

#endif // CALCULATOR_H

#include "Calculator.h"

double Calculator::Calc(double num1, char op, double num2) {
    switch(op) {
        case '+':
            return add(num1, num2);
        case '-':
            return sub(num1, num2);
        case '*':
            return mult(num1, num2);
        case '/':
            return divi(num1, num2);
        default:
            return 0.0;
    }
}

double Calculator::add(double num1, double num2) {
    return num1 + num2;
}

double Calculator::sub(double num1, double num2) {
    return num1 - num2;
}

double Calculator::mult(double num1, double num2) {
    return num1 * num2;
}

double Calculator::divi(double num1, double num2) {
    if(num2 == 0) return 0.0;
    return num1 / num2;
}