编写一个memmove函数，实现内存拷贝

void* my_memmove(void *des,const void *src,int size)
{
	char *Des=(char*)des;
	char *Src=(char*)src;
	assert(Des && Src);
	int len=strlen(Src);
	assert(size >0 && size <len);
	if(Des < Src || Des+size > Src+len)
	{
		while(size)
		{
			*Des++=*Src++;
			--size;
		}
	}else
	{
		Des=Des+size;
		Src=Src+size;
		while(size)
		{
			*Des--=*Src--;
			--size;
		}
	}
	return des;
}