How to use a recursive function in the C language to calculate the factorial of n?

Here is an example code using a recursive function in C language to calculate the factorial of n.

#include <stdio.h>

// 声明递归函数
unsigned long long factorial(int n);

int main() {
    int n;
    printf("请输入一个正整数n：");
    scanf("%d", &n);
    
    // 调用递归函数求解阶乘
    unsigned long long result = factorial(n);
    
    printf("%d的阶乘为%llu\n", n, result);
    
    return 0;
}

// 定义递归函数
unsigned long long factorial(int n) {
    if (n <= 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

In the above code, a recursive function called factorial is used to calculate the factorial of n. The function first checks for the termination condition that if n is less than or equal to 1, it returns 1 to indicate the end of the factorial. Otherwise, it continues to call the recursive function with n-1 as the parameter, multiplying the result with n to get the factorial of n.

In the main function, it first reads a positive integer n input by the user, then calls the recursive function factorial to calculate the factorial of n, and finally displays the result on the screen.