C语言如何计算阶乘？

在C语言中，计算阶乘（factorial）可以通过递归或循环的方式实现。这里我会给你两种常见的方法：

**1. 递归法：**

#include <stdio.h>

unsigned long long factorial(int n) {
    if (n == 0 || n == 1) { // 阶乘的基础情况
        return 1;
    } else {
        return n * factorial(n - 1); // 递归调用
    }
}

int main() {
    int num;
    printf("Enter a non-negative integer: ");
    scanf("%d", &num);
    
    if (num >= 0) {
        unsigned long long result = factorial(num);
        printf("The factorial of %d is %llu\n", num, result);
    } else {
        printf("Invalid input. Please enter a non-negative number.\n");
    }

    return 0;
}

**2. 循环法：**

#include <stdio.h>

unsigned long long factorial(int n) {
    unsigned long long fact = 1;
    for (int i = 1; i <= n; i++) {
        fact *= i;
    }
    return fact;
}

int main() {
    int num;
    printf("Enter a non-negative integer: ");
    scanf("%d", &num);

    if (num >= 0) {
        unsigned long long result = factorial(num);
        printf("The factorial of %d is %llu\n", num, result);
    } else {
        printf("Invalid input. Please enter a non-negative number.\n");
    }

    return 0;
}

这两种方法都可以有效地计算非负整数的阶乘，但请注意，对于较大的数值，递归可能会导致栈溢出，因此循环法更为稳健。