kmp算法参考
http://mp.weixin.qq.com/s?__biz=MjM5NTY1MjY0MQ==&mid=2650749474&idx=3&sn=a4e34d48936c41f999b6fec372aab9ee&chksm=befeaf6c8989267a60fe9082cb6c72fc1ccb2eb53a10ccf141fc18e058ce8051c4fbb73cff4f&mpshare=1&scene=23&srcid=&sharer_sharetime=1583551818919&sharer_shareid=7de6be81b4103b8673bc7e7c26a194c7#rd