原

#include <stdio.h>
int main(){
    int n;
    int sum = 1;
    printf("输入数字：");
    scanf("%d", &n);
    for (int i = 1;i <= n ;i++ ) {
        sum *= i;
    }
    printf("%d的阶乘为%d\n", n, sum);

    return 0;
}

优化

#include <stdio.h>

int main() {
    int num;
    int factorial = 1;
    
    printf("输入数字：");
    scanf("%d", &num);
    
    if (num <= 0) {
        printf("无效输入！请输入一个正整数。\n");
        return 1;
    }
    
    for (int i = 1; i <= num; i++) {
        factorial *= i;
    }
    
    printf("%d的阶乘为%d\n", num, factorial);
    
    return 0;
}

递归

#include <stdio.h>

int factorial(int n) {
    if (n == 0 || n == 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

int main() {
    int num;
    
    printf("输入数字：");
    scanf("%d", &num);
    
    if (num < 0) {
        printf("无效输入！请输入一个非负整数。\n");
        return 1;
    }
    
    int result = factorial(num);
    printf("%d的阶乘为%d\n", num, result);
    
    return 0;
}