L819.java

import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.function.Predicate;
import java.util.stream.Collectors;

class Solution819 {
    class Solution {
        /**
         * 819. Most Common Word https://leetcode.com/problems/most-common-word/description/
         *
         * @timeComplexity O(n) where n is the length of string
         * @spaceComplexity max(O ( k), O(l)) where k is the number of unique words in the string, l is the list of banned words
         * @param paragraph String
         * Paragraph to scan.
         * @param banned String[]
         * List of banned words
         * @return int[] containing distances
         */
        // Predicate to check whether a word ends with punctuation symbol
        private final Predicate<String> puncChecker = (str) -> ",.!?';".chars().anyMatch(p -> str.charAt(str.length() - 1) == p);

        public String mostCommonWord(String paragraph, String[] banned) {
            // Populate a set of banned words
            Set<String> bannedSet = new HashSet<>(Arrays.asList(banned).stream().map(String::toLowerCase).collect(Collectors.toList()));
            Map<String, Integer> countMap = new HashMap<>();

            String[] words = paragraph.toLowerCase().split(" ");
            // Scan words and update their frequencies in map
            for (int i = 0; i < words.length; i++) {
                String word = words[i];

                // Remove last char if word ends with punctuation
                if (puncChecker.test(word)) {
                    word = word.substring(0, word.length() - 1);
                }
                if (bannedSet.contains(word)) {
                    continue;
                }
                if (countMap.containsKey(word)) {
                    countMap.put(word, countMap.get(word) + 1);
                } else {
                    countMap.put(word, 1);
                }
            }
            // Find the max occuring word
            int max = 0;
            String maxWord = "";
            for (Map.Entry<String, Integer> entry : countMap.entrySet()) {
                if (entry.getValue() > max) {
                    max = entry.getValue();
                    maxWord = entry.getKey();
                }
            }
            return maxWord;
        }
    }
}