C++函数问题的编写

问题遇到的现象和发生背景 C++编程序写函数

用代码块功能插入代码，请勿粘贴截图 编写函数fac现n!的计算,并求解和输出1到4的求和的值。

我的解答思路和尝试过的方法

```c++

#include
using namespace std;
int main()
{
    int fac(int);
    int a, b, c, d, sum = 0;
    cout << "enter a,b,c,d:";
    cin >> a >> b >> c >> d;
    sum = sum + fac(a) + fac(b) + fac(c) + fac(d);
    cout << a << "!+" << b << "!+" << c << "!+" << d << "!=" << sum << endl;
    system("pause");
    return 0;
}
int fac(int n)
{
    int f = 1;
    for (int i = 1; i <= n; i++)
        f = f*i;
    return f;
}
![img](https://img-mid.csdnimg.cn/release/static/image/mid/ask/930228307766149.jpg "#left")

###### 我想要达到的结果
![img](https://img-mid.csdnimg.cn/release/static/image/mid/ask/947838307766194.jpg "#left")

#include<iostream>
using namespace std;
int main()
{
    int fac(int);
    int a, b, c, d, sum = 0;
    cout << "enter a,b,c,d:";
    cin >> a >> b >> c >> d;
    sum = sum + fac(a) + fac(b) + fac(c) + fac(d);
    cout << a << "!+" << b << "!+" << c << "!+" << d << "!=" << sum << endl;
    system("pause");
    return 0;
}

int fac(int n)
{
    if (n < 0) return 0;
    if (n == 0 || n == 1)return 1;
    if (n > 1)
    {
        return n * fac(n - 1);
    }
}