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更改有两个地方
1 把两个经常出现的操作(删除当前节点以及替换头结点)封装成helper函数, 提升阅读体验
2 原来检验一个key是否在字典中, 用的其实是线性查找时间, 也不够pythonic, 改为更快更pythonic的写法