127. Word Ladder

class Solution {
    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        if (wordList == null || wordList.size() == 0) {
            return 0;
        }
        Set<String> wordSet = new HashSet<>();
        for (String word : wordList) {
            wordSet.add(word);
        }
        
        if (!wordList.contains(endWord)) {
            return 0;
        }
        
        if (wordList.contains(beginWord)) {
            wordList.remove(beginWord);
        }
        
        Queue<String> queue = new LinkedList<>();
        queue.offer(beginWord);
        
        int count = 1;
        while (!queue.isEmpty()) {
            int size = queue.size();
            
            for (int i = 0; i < size; i++) {
                String word = queue.poll();
                char[] charArray = word.toCharArray();
                for (int j = 0; j < beginWord.length(); j++) {
                    char cur = charArray[j];
                    for (char c = 'a'; c <= 'z'; c++) {
                        if (c == charArray[j]) {
                            continue;
                        }
                        charArray[j] = c;
                        String newWord = new String(charArray);
                        if (newWord.equals(endWord)) {
                            return count + 1;
                        }
                        if (wordSet.contains(newWord)) { // here is important
                            queue.offer(newWord);
                            wordSet.remove(newWord); // this step is important
                        }
                        charArray[j] = cur;
                    }
                }
                
            }
            count ++; 
        }
        return 0;
    }
}