126. Word Ladder II

class Solution {
    public List<List<String>> findLadders(String beginWord, String endWord, List<String> wordList) {
        List<List<String>> res=new ArrayList<List<String>>();
        Set<String> dict=new HashSet<String>(wordList);
        if(!dict.contains(endWord))
            return res;
        Set<String> set1=new HashSet<String>(Arrays.asList(beginWord));
        Set<String> set2=new HashSet<String>(Arrays.asList(endWord));
        Map<String, List<String>> map=new HashMap<String, List<String>>();
        buildmap(set1,set2,dict,map,false);
        generatePath(new ArrayList<String>(Arrays.asList(beginWord)),res,map,endWord);
        return res;
    }
    private void buildmap(Set<String> set1, Set<String> set2, Set<String> dict, Map<String, List<String>> map, boolean flip){
        if(set1.isEmpty())
            return;
        if(set1.size()>set2.size())
        {
            buildmap(set2, set1, dict, map, !flip);
            return;
        }
        dict.removeAll(set1);
        Set<String> set3=new HashSet<String>();
        boolean found=false;
        for(String w:set1)
        {
            for(int i=0;i<w.length();i++)
            {
                char[] arr=w.toCharArray();
                for(char ch='a';ch<='z';ch++)
                {
                    arr[i]=ch;
                    String nw=new String(arr);
                    String key=flip?nw:w;
                    String val=flip?w:nw;
                    if(!map.containsKey(key))
                        map.put(key, new ArrayList<String>());
                    if(set2.contains(nw))
                    {
                        found=true;
                        map.get(key).add(val);
                    }
                    if(found==false&&dict.contains(nw))
                    {
                        map.get(key).add(val);
                        set3.add(nw);
                    }
                }
            }
        }
        if(found==false)
            buildmap(set2,set3,dict,map,!flip);
    }
    private void generatePath(List<String> path, List<List<String>> res, Map<String, List<String>> map, String endWord){
        String word=path.get(path.size()-1);
        if(word.equals(endWord))
        {
            if(res.size()>0&&path.size()<res.get(0).size())
                res.clear();
            if(res.size()==0||path.size()==res.get(0).size())
                res.add(new ArrayList<String>(path));
            return;
        }
        if(!map.containsKey(word))
            return;
        for(String w:map.get(word))
        {
            if(path.contains(w))
                continue;
            path.add(w);
            generatePath(path,res,map,endWord);
            path.remove(path.size()-1);
        }
    }
}