1 题目理解

题目要求和127是一样的。返回值不一样。返回值要求把最短路径的，具体路径输出。
Input:
beginWord = “hit”,
endWord = “cog”,
wordList = [“hot”,“dot”,“dog”,“lot”,“log”,“cog”]

Output:
[
[“hit”,“hot”,“dot”,“dog”,“cog”],
[“hit”,“hot”,“lot”,“log”,“cog”]
]

2 BFS

因为要返回具体的路径。需要修改的地方有2个。
一是不能再有虚拟节点，需要两两比对单词不相同的位数有几个，判断是否可以添加边。
二是，队列中的值不再是节点本身，还需要把到达这个节点的具体路径记录下来。

class Solution {public List<List<String>> findLadders(String beginWord, String endWord, List<String> wordList) {List<List<String>> answer = new ArrayList<List<String>>();if(wordList==null || !wordList.contains(endWord)){return answer;}if(!wordList.contains(beginWord)){wordList.add(beginWord);}Map<String,List<String>> allComboDict = addWord(wordList);Queue<Pair> queue = new LinkedList<Pair>();List<String> beginList = new ArrayList<String>();beginList.add(beginWord);queue.offer(new Pair(beginWord,beginList));Map<String,Integer> visited = new HashMap<String,Integer>();visited.put(beginWord,1);while(!queue.isEmpty()){Pair pair = queue.poll();String node = pair.word;List<String> list = pair.list;if(node.equals(endWord)){answer.add(list);}else{if(allComboDict.get(node)!=null){for(String toNode: allComboDict.get(node)){if(!visited.containsKey(toNode) || visited.get(toNode)>=list.size()+1){List<String> tmp = new ArrayList<String>(list);tmp.add(toNode);queue.offer(new Pair(toNode,tmp));visited.put(toNode,tmp.size());}}}}}return answer;}private  Map<String,List<String>> addWord(List<String> wordList){Map<String,List<String>> allComboDict = new HashMap<String,List<String>>();for(int i=0;i<wordList.size();i++){for(int j=i+1;j<wordList.size();j++){if(transform(wordList.get(i),wordList.get(j))){List<String> list = allComboDict.getOrDefault(wordList.get(j),new ArrayList<String>());list.add(wordList.get(i));allComboDict.put(wordList.get(j),list);List<String> list2 = allComboDict.getOrDefault(wordList.get(i),new ArrayList<String>());list2.add(wordList.get(j));allComboDict.put(wordList.get(i),list2);}}}System.out.println(allComboDict);return allComboDict;}private boolean transform(String a, String b){int diff = 0;for(int i=0;i<a.length();i++){if(a.charAt(i)!=b.charAt(i)){diff++;}}return diff == 1;}class Pair{private String word;private List<String> list;public Pair(String word,List<String> list){this.word = word;this.list = list;}}
}