https://files.cnblogs.com/files/heshiye/%E7%BD%91%E7%BB%9C%E7%A7%91%E5%AD%A6%E5%AF%BC%E8%AE%BA%E7%AC%AC%E4%B8%80%E6%AC%A1%E4%BD%9C%E4%B8%9A.ppt
