public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        if (wordList == null || wordList.size() == 0 || !wordList.contains(endWord)) return 0;
        Queue<String> queue = new LinkedList<>();
        Set<String> words = new HashSet<>(wordList);
        queue.offer(beginWord);
        int steps = 1;
        while (!queue.isEmpty()) {
            int size = queue.size();
            steps++;
            for (int i = 0; i < size; i++) {
                String cur = queue.poll();
                List<String> candidates = transform(words, cur);
                for (String candidate : candidates) {
                    if (candidate.equals(endWord)) return steps;
                    queue.offer(candidate);
                }
            }
        }
        return 0;
    }


    /**
     * 生成目标的word的所有潜在的word，
     * 如hit -->ait bit ...zit但是排除了hit本身
     * hit --> hat hbt... hzt但是排除了hit本身
     * words 含有的上面生成的潜在的word进行收集
     *
     * @param words
     * @param word
     * @return
     */
    private List<String> transform(Set<String> words, String word) {
        List<String> resList = new ArrayList<>();
        StringBuffer sb = new StringBuffer(word);
        for (int i = 0; i < sb.length(); i++) {
            char tmp = sb.charAt(i);//记录下索引位置下的char，下面的for loop中会剔除掉这个
            for (char c = 'a'; c <= 'z'; c++) {
                if (tmp == c) continue;//word本身
                sb.setCharAt(i, c);//改变i的值
                String canditate = sb.toString();
                //如果words含有canditate，将其加入到结果集中
                if (words.remove(canditate)) resList.add(canditate);
            }
            sb.setCharAt(i, tmp);//结束本轮loop后，恢复原样
        }

        return resList;
    }

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

            Set<String> wordSet = new HashSet<>(wordList);
            if (!wordSet.contains(endWord)) return 0;
            Set<String> beginSet = new HashSet<>();
            Set<String> endSet = new HashSet<>();
            Set<String> visitdSet = new HashSet<>();
            beginSet.add(beginWord);
            endSet.add(endWord);
            int steps = 1;
            while (!beginSet.isEmpty() && !endSet.isEmpty()) {
                if (beginSet.size() > endSet.size()) {
                    Set<String> tmpSet = beginSet;
                    beginSet = endSet;
                    endSet = tmpSet;
                }
                Set<String> tmpSet = new HashSet<>();
                System.out.printf("beginSet:%s\n", JSON.toJSONString(beginSet));
                System.out.printf("endSet:%s\n", JSON.toJSONString(endSet));
                for (String word : beginSet) {
                    char[] chas = word.toCharArray();
                    for (int i = 0; i < chas.length; i++) {
                        char src = chas[i];
                        for (char c = 'a'; c <= 'z'; c++) {
                            if (c == src) continue;
                            chas[i] = c;
                            String nextWord = String.valueOf(chas);
                            //已经找到了
                            if (endSet.contains(nextWord)) {
                                return steps + 1;
                            }
                            //没有被访问过，且单词在wordSet中，添加进tmpSet ，本轮结束后赋值给beginSet
                            if (!visitdSet.contains(nextWord) && wordSet.contains(nextWord)) {
                                tmpSet.add(nextWord);
                                visitdSet.add(nextWord);
                            }
                        }
                        chas[i] = src;
                    }
                }
                beginSet = tmpSet;
                steps++;
            }
            return 0;
        }