实现 memcpy

void * Memcpy(void * src, void * des, int len)
{
    char * pSrc, *pDes;
    if (NULL == des || NULL == src)
        return NULL;
    if (src < des && (char*)src + len  >(char*)des)
    {
        pSrc = (char*)src + len - 1;
        pDes = (char*)des + len - 1;
        while (len--)
        {
            *pDes-- = *pSrc--;
        }
    }
    else
    {
        pSrc = (char*)src;
        pDes = (char*)des;
        while (len--)
        {
            *pDes++ = *pSrc++;
        }
    }

    return des;
}