void *memcpy(void *dst, const void *src, size_t len)  
{  
    if(NULL == dst || NULL == src){  
        return NULL;  
    }  
      
    void *ret = dst;  
      
    if(dst <= src || (char *)dst >= (char *)src + len){  
        //没有内存重叠，从低地址开始复制  
        while(len--){  
            *(char *)dst = *(char *)src;  
            dst = (char *)dst + 1;  
            src = (char *)src + 1;  
        }  
    }else{  
        //有内存重叠，从高地址开始复制  
        src = http://www.mamicode.com/(char *)src + len - 1;  
        dst = (char *)dst + len - 1;  
        while(len--){  
            *(char *)dst = *(char *)src;  
            dst = (char *)dst - 1;  
            src = (char *)src - 1;  
        }  
    }  
    return ret;  
}