题目：

题解：

class Solution {Map<String, Integer> wordId = new HashMap<String, Integer>();List<List<Integer>> edge = new ArrayList<List<Integer>>();int nodeNum = 0;public int ladderLength(String beginWord, String endWord, List<String> wordList) {for (String word : wordList) {addEdge(word);}addEdge(beginWord);if (!wordId.containsKey(endWord)) {return 0;}int[] disBegin = new int[nodeNum];Arrays.fill(disBegin, Integer.MAX_VALUE);int beginId = wordId.get(beginWord);disBegin[beginId] = 0;Queue<Integer> queBegin = new LinkedList<Integer>();queBegin.offer(beginId);int[] disEnd = new int[nodeNum];Arrays.fill(disEnd, Integer.MAX_VALUE);int endId = wordId.get(endWord);disEnd[endId] = 0;Queue<Integer> queEnd = new LinkedList<Integer>();queEnd.offer(endId);while (!queBegin.isEmpty() && !queEnd.isEmpty()) {int queBeginSize = queBegin.size();for (int i = 0; i < queBeginSize; ++i) {int nodeBegin = queBegin.poll();if (disEnd[nodeBegin] != Integer.MAX_VALUE) {return (disBegin[nodeBegin] + disEnd[nodeBegin]) / 2 + 1;}for (int it : edge.get(nodeBegin)) {if (disBegin[it] == Integer.MAX_VALUE) {disBegin[it] = disBegin[nodeBegin] + 1;queBegin.offer(it);}}}int queEndSize = queEnd.size();for (int i = 0; i < queEndSize; ++i) {int nodeEnd = queEnd.poll();if (disBegin[nodeEnd] != Integer.MAX_VALUE) {return (disBegin[nodeEnd] + disEnd[nodeEnd]) / 2 + 1;}for (int it : edge.get(nodeEnd)) {if (disEnd[it] == Integer.MAX_VALUE) {disEnd[it] = disEnd[nodeEnd] + 1;queEnd.offer(it);}}}}return 0;}public void addEdge(String word) {addWord(word);int id1 = wordId.get(word);char[] array = word.toCharArray();int length = array.length;for (int i = 0; i < length; ++i) {char tmp = array[i];array[i] = '*';String newWord = new String(array);addWord(newWord);int id2 = wordId.get(newWord);edge.get(id1).add(id2);edge.get(id2).add(id1);array[i] = tmp;}}public void addWord(String word) {if (!wordId.containsKey(word)) {wordId.put(word, nodeNum++);edge.add(new ArrayList<Integer>());}}
}