strcpy和memset的引用
int main()
{
char arr1[] ="hello";
char arr2[20] = "#########";
//arr2[]copy时把arr1[]里的结束\0也复制过来了，所以打印遇到\0结束打印
strcpy(arr2, arr1);
memset(arr1, '#', 2);//用#替换arr1里前两个字符
printf("%s\n", arr1);
printf("%s", arr2);
return 0;
}