1、先给出字符串：
a = "If we give a paragraph of English text, we want to get the number of times each letter appears, we need to use the function: LetterCounts.This article introduces the usage of this function."
2、统计字母频率：
LetterCounts[a]
出现次数最多的是字母e。
3、出现次数最多的双字母是ti：
LetterCounts[a, 2]
4、the是出现次数最多的三字母：
LetterCounts[a, 3]
5、出现次数最多的字符是空格：
CharacterCounts[a]
6、出现次数最多的双字符是"e "：
Reverse[Sort@CharacterCounts[a, 2]]
7、出现次数最多的单词是we、of、the：
Reverse[Sort@WordCounts[a]]
