Given two words (beginWord and endWord), and a dictionary's word list, find the length of shortest transformation sequence from beginWord to endWord, such that: Only one letter can be changed at a time. Each transformed word must exist in the word list. Note that beginWord is not a transformed word.

WordLength.java

public static int ladderLength(String beginWord, String endWord, List<String> wordList) {
    int count = 1;
    if(beginWord.equals(endWord))
        return count;

    Queue<String> queue = new LinkedList<>();
    Set<String> set = new HashSet<String>(wordList); // contains() is O(1). For ArrayList it's O(n)
    queue.add(beginWord);
    set.remove(beginWord);

    while(queue.size() > 0) {
        count++;
        int size = queue.size();
        for(int i = 0; i < size; i++) {                // to keep a track of depth
            String polled = queue.poll();           

            for(int j = 0; j < polled.length(); j++){ // to iterate each character
                char[] element = polled.toCharArray();

                for(char k = 'a'; k <= 'z'; k++) {  // to try all possibilities
                    element[j] = k;
                    String word = new String(element);

                    if(set.contains(word) && word.equals(endWord))
                        return count;
                    if(set.remove(word))
                        queue.add(word);
                }
            }
        }
    }


    return 0;
}