方法1

#include<stdio.h>
int main()
{
	int n = 0;
	printf("请输入n的值:");
	scanf_s("%d",&n);
	int sz = 1;//sz!=0
	int i = 1;//循环里面i!=0,否则循环没有意义
	for ( i = 1; i <=n; i++)
	{
		sz *= i;
	}
	printf("阶乘为:%d\n",sz);
	return 0;
}

运行结果

方法2

#include<stdio.h>
int main()
{
	//n的阶乘
	int n;
	printf("请输入n的值:");
	while (scanf_s("%d",&n))
	{
		int sz = 1;
		int i = 0;
		for ( i = 1; i <= n; i++)
		{
			sz = sz*i;
		}
		printf("阶乘为:%d\n", sz);
	}
	return 0;
}

运行结果 

希望可以帮助到有需要的人！！！