Some are numeric and others contain letters.
有的是数字，有的则包含字母。
If you expect a string to contain only ASCII letters, be sure that it does.
如果期望字符串只包含ASCII字母，请确定它确实是这样。
They can contain both letters and Numbers, but they can't begin with a number.
包含字母或者数字都行，但是不能用数字来开头。
Police said the letters were safe and did not contain threats to lawmakers.
警察说这些信件是安全的，不含有对议员的威胁。
Note: You may assume the string contain only lowercase letters.
注意：假定字符串中只包含小写字母。
You may assume that each word will contain only lower case letters.
你可以假设每一个单词只包含小写字母。
You may assume that each word will contain only lower case letters.
你可以假设每一个单词只包含小写字母。
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