LeetCode 819. Most Common Word (最常见的单词)

Given a paragraph and a list of banned words, return the most frequent word that is not in the list of banned words.  It is guaranteed there is at least one word that isn't banned, and that the answer is unique.

Words in the list of banned words are given in lowercase, and free of punctuation.  Words in the paragraph are not case sensitive.  The answer is in lowercase.

Example:
Input: 
paragraph = "Bob hit a ball, the hit BALL flew far after it was hit."
banned = ["hit"]
Output: "ball"
Explanation: 
"hit" occurs 3 times, but it is a banned word.
"ball" occurs twice (and no other word does), so it is the most frequent non-banned word in the paragraph. 
Note that words in the paragraph are not case sensitive,
that punctuation is ignored (even if adjacent to words, such as "ball,"), 
and that "hit" isn't the answer even though it occurs more because it is banned.

Note:

• 1 <= paragraph.length <= 1000.
• 1 <= banned.length <= 100.
• 1 <= banned[i].length <= 10.
• The answer is unique, and written in lowercase (even if its occurrences in paragraph may have uppercase symbols, and even if it is a proper noun.)
• paragraph only consists of letters, spaces, or the punctuation symbols !?',;.
• Different words in paragraph are always separated by a space.
• There are no hyphens or hyphenated words.
• Words only consist of letters, never apostrophes or other punctuation symbols.

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题目标签：String

　　题目给了我们一段paragraph，还有 banned words，让我们找出 在 paragraph 里出现最多的 non-banned word。

　　主要思想是，开始要把paragraph 都变成小写，然后利用 regex split 成 string array；

　　接着把banned words 放入 HashSet；

　　把 non-banned words 放入 HashMap，在这一过程中，可以保留 出现次数最多的 word，不需要在多一个loop 来找出 max。

　　!s.equals("") 的作用是因为 regex 会造成 “” 的情况，所以要跳过。

　　具体请看code。

　　

Java Solution:

Runtime beats  (no enough accepted submissions)% 

完成日期：05/06/2018

关键词：HashSet; HashMap; Regex

关键点：用 Regex 来filter out string

class Solution 
{
    public String mostCommonWord(String paragraph, String[] banned) 
    {
        HashSet<String> bannedSet = new HashSet<>();
        HashMap<String, Integer> wordsMap = new HashMap<>();
        String mcw = "";
        int mcwCount = -1;
        
        // filter spaces and punctuation
        String[] wordsArr = paragraph.toLowerCase().split(" |!|\?|'|,|;|\."); 
        
        // put banned words into hash set
        for(String s: banned)
            bannedSet.add(s);
    
        // add and count non-banned words into wordsMap
        for(String s: wordsArr)
        {
            if(!bannedSet.contains(s) && !s.equals(""))
            {
                wordsMap.put(s, wordsMap.getOrDefault(s, 0) + 1);
            
                int count = wordsMap.get(s);
                // keep tracking the most common word
                if(count > mcwCount)
                {
                    mcw = s;
                    mcwCount = count;
                }
            }
            
        }
                
        return mcw;
        
    }
}

参考资料：n/a

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