126. Word Ladder II

題目：
Given two words (beginWord and endWord), and a dictionary's word list, find all shortest transformation sequence(s) from beginWord to endWord, such that:

Only one letter can be changed at a time
Each intermediate word must exist in the word list
For example,

Given:
beginWord = "hit"
endWord = "cog"
wordList = ["hot","dot","dog","lot","log"]
Return
[

["hit","hot","dot","dog","cog"],
["hit","hot","lot","log","cog"]

]

解答：

public class Solution {
    List<List<String>> result;
    List<String> list;
    Map<String, List<String>> map;
    public List<List<String>> findLadders(String beginWord, String endWord, Set<String> wordList) {
        result = new ArrayList<List<String>>();
        if (wordList.size() == 0) return result;
        
        list = new LinkedList<String>();
        map = new HashMap<String, List<String>>();
        int curt = 1, next = 0;
        boolean found = false;
        Set<String> unvisited = new HashSet<String>(wordList);
        Set<String> visited = new HashSet<String>();
        Queue<String> queue = new ArrayDeque<String>();
        
        queue.add(beginWord);
        unvisited.add(endWord);
        unvisited.remove(beginWord);
        //BFS
        while (!queue.isEmpty()) {
            String word = queue.poll();
            curt--;
            for (int i = 0; i < word.length(); i++) {
                StringBuilder sb = new StringBuilder(word);
                for (char ch = 'a'; ch <= 'z'; ch++) {
                    sb.setCharAt(i, ch);
                    String newWord = sb.toString();
                    if (unvisited.contains(newWord)) {
                        if (visited.add(newWord)) {
                            next++;
                            queue.add(newWord);
                        }
                        
                        if (!map.containsKey(newWord)) {
                            map.put(newWord, new LinkedList<String>());
                        }
                        map.get(newWord).add(word);
                        
                        if (newWord.equals(endWord) && !found) found = true;
                    }
                }
            }
            if (curt == 0) {
                if (found) break;
                curt = next;
                next = 0;
                unvisited.removeAll(visited);
                visited.clear();
            }
        }
        
        backTrace(endWord, beginWord);
        return result;
    }
    
    public void backTrace(String word, String beginWord) {
        if (word.equals(beginWord)) {
            list.add(0, beginWord);
            result.add(new ArrayList<String>(list));
            list.remove(0);
            return;
        }
        list.add(0, word);
        if (map.get(word) != null) {
            for (String s : map.get(word)) {
                backTrace(s, beginWord);
            }
        }
        list.remove(0);
    }
}