Design a method to find the frequency of occurrences of any given word in a book. What if we were running this algorithm multiple times?

You should implement following methods:

• WordsFrequency(book) constructor, parameter is a array of strings, representing the book.
• get(word) get the frequency of word in the book. 

Example:

WordsFrequency wordsFrequency = new WordsFrequency({"i", "have", "an", "apple", "he", "have", "a", "pen"});

wordsFrequency.get("you"); //returns 0，"you" is not in the book

wordsFrequency.get("have"); //returns 2，"have" occurs twice in the book

wordsFrequency.get("an"); //returns 1

wordsFrequency.get("apple"); //returns 1

wordsFrequency.get("pen"); //returns 1

Note:

• There are only lowercase letters in book[i].
• 1 <= book.length <= 100000
• 1 <= book[i].length <= 10
• get function will not be called more than 100000 times.