Most Common Word
<p>Given a string <code>paragraph</code> and a string array of the banned words <code>banned</code>, return <em>the most frequent word that is not banned</em>. It is <strong>guaranteed</strong> there is <strong>at least one word</strong> that is not banned, and that the answer is <strong>unique</strong>.</p>

<p>The words in <code>paragraph</code> are <strong>case-insensitive</strong> and the answer should be returned in <strong>lowercase</strong>.</p>

<p><strong>Note</strong> that words can not contain punctuation symbols.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> paragraph = &quot;Bob hit a ball, the hit BALL flew far after it was hit.&quot;, banned = [&quot;hit&quot;]
<strong>Output:</strong> &quot;ball&quot;
<strong>Explanation:</strong> 
&quot;hit&quot; occurs 3 times, but it is a banned word.
&quot;ball&quot; 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 &quot;ball,&quot;), 
and that &quot;hit&quot; isn&#39;t the answer even though it occurs more because it is banned.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> paragraph = &quot;a.&quot;, banned = []
<strong>Output:</strong> &quot;a&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= paragraph.length &lt;= 1000</code></li>
	<li>paragraph consists of English letters, space <code>&#39; &#39;</code>, or one of the symbols: <code>&quot;!?&#39;,;.&quot;</code>.</li>
	<li><code>0 &lt;= banned.length &lt;= 100</code></li>
	<li><code>1 &lt;= banned[i].length &lt;= 10</code></li>
	<li><code>banned[i]</code> consists of only lowercase English letters.</li>
</ul>
Array; Hash Table; String; Counting
import java.util.regex.Matcher;
import java.util.regex.Pattern;

class Solution {
    private static Pattern pattern = Pattern.compile("[a-z]+");

    public String mostCommonWord(String paragraph, String[] banned) {
        Set<String> bannedWords = new HashSet<>();
        for (String word : banned) {
            bannedWords.add(word);
        }
        Map<String, Integer> counter = new HashMap<>();
        Matcher matcher = pattern.matcher(paragraph.toLowerCase());
        while (matcher.find()) {
            String word = matcher.group();
            if (bannedWords.contains(word)) {
                continue;
            }
            counter.put(word, counter.getOrDefault(word, 0) + 1);
        }
        int max = Integer.MIN_VALUE;
        String ans = null;
        for (Map.Entry<String, Integer> entry : counter.entrySet()) {
            if (entry.getValue() > max) {
                max = entry.getValue();
                ans = entry.getKey();
            }
        }
        return ans;
    }
}