计算一个数字阶乘值

正整数n的阶乘由n!表示 并定义如下：

n! = 1 x  2 x  ..... x  n

例如：

0! = 1                        
1! = 1
2! = 1 x 2 = 2
3! = 1 x 2 x 3 = 6

示例代码：

#include <stdio.h>

int main(){
     int counter=  10;
     unsigned long int result = 1;
     char ch;

     for (int i = 1; i <= counter; i++) {
        result = result * i;
     }

     printf("required factorial is: %lu\n", result);

     return(0);
}