程序员面试金典-面试题 16.02. 单词频率

题目：

设计一个方法，找出任意指定单词在一本书中的出现频率。

你的实现应该支持如下操作：

WordsFrequency(book)构造函数，参数为字符串数组构成的一本书
get(word)查询指定单词在数中出现的频率
示例：

WordsFrequency wordsFrequency = new WordsFrequency({"i", "have", "an", "apple", "he", "have", "a", "pen"});
wordsFrequency.get("you"); //返回0，"you"没有出现过
wordsFrequency.get("have"); //返回2，"have"出现2次
wordsFrequency.get("an"); //返回1
wordsFrequency.get("apple"); //返回1
wordsFrequency.get("pen"); //返回1
提示：

book[i]中只包含小写字母
1 <= book.length <= 100000
1 <= book[i].length <= 10
get函数的调用次数不会超过100000

分析：

利用HashMap存储book出现的频次即可。

程序：

class WordsFrequency {

    public WordsFrequency(String[] book) {
        dict = new HashMap<>();
        for(String bookName:book){
            int freq = dict.getOrDefault(bookName, 0) + 1;
            dict.put(bookName, freq);
        }
    }
    
    public int get(String word) {
        return dict.getOrDefault(word, 0);
    }
    private HashMap<String, Integer> dict;
}

/**
 * Your WordsFrequency object will be instantiated and called as such:
 * WordsFrequency obj = new WordsFrequency(book);
 * int param_1 = obj.get(word);
 */