Solution.java

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;
    }
}