public int ladderLength(String beginWord, String endWord, List<String> wordList) {

int L = beginWord.length();

Map<String, List<String>> allComboDict = new HashMap<>();

wordList.forEach(

word -> {

for (int i = 0; i < L; i++) {

String newWord = word.substring(0, i) + '*' + word.substring(i + 1, L);

List<String> transformations = allComboDict.getOrDefault(newWord, new ArrayList<>());

transformations.add(word);

allComboDict.put(newWord, transformations);

}

});

Queue<Pair<String, Integer>> Q = new LinkedList<>();

Q.add(new Pair(beginWord, 1));

Map<String, Boolean> visited = new HashMap<>();

visited.put(beginWord, true);

while (!Q.isEmpty()) {

Pair<String, Integer> node = Q.remove();

String word = node.getKey();

int level = node.getValue();

for (int i = 0; i < L; i++) {

String newWord = word.substring(0, i) + '*' + word.substring(i + 1, L);

for (String adjacentWord : allComboDict.getOrDefault(newWord, new ArrayList<>())) {

if (adjacentWord.equals(endWord)) {

return level + 1;

}

if (!visited.containsKey(adjacentWord)) {

visited.put(adjacentWord, true);

Q.add(new Pair(adjacentWord, level + 1));

}

}

}

}

return 0;

}