Given an array of strings, group anagrams together. https://leetcode.com/explore/challenge/card/30-day-leetcoding-challenge/528/week-1/3288/

GroupAnagrams.java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class GroupAnagrams {
	public static void main(String[] args) {
		GroupAnagrams problem = new GroupAnagrams();
		List<List<String>> result1 = problem.groupAnagrams(new String[] { "eat", "tea", "tan", "ate", "nat", "bat" });
		System.out.println("Expected: 6, \n"
				+ "Result: " + result1);
	}

	private static final char PRIMES[] = { 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71,
			73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181,
			191, 193, 197, 199 };

	public List<List<String>> groupAnagrams(String[] wordsArray) {
		Map<Integer, List<String>> anagramWords = new HashMap<>();
		for (String word : wordsArray) {
			Integer key = calculateKey(word);
			List<String> wordList = anagramWords.get(key);
			if (wordList == null) {
				wordList = new ArrayList<String>();
				anagramWords.put(key, wordList);
			}
			wordList.add(word);
		}
		// return anagramWords.values().stream().collect(Collectors.toCollection(ArrayList::new));
		return anagramWords.values().stream().collect(Collectors.toList());
	}

	private Integer calculateKey(String word) {

		// We know word is all lowercase. Otherwise we can convert it to lowercase.
		// word = word.toLowerCase();

		int key = 1;
		for (char letter : word.toCharArray()) {
			key *= PRIMES[letter - 'a'];
		}
		return key;
	}
}