word ladder II

public List<List<String>> findLadders(String start, String end, List<String> wordList) {
        Set<String> dict = new HashSet<>();
        for(String w : wordList)
            dict.add(w);
        
        int len = start.length();
        Set<String> visited = new HashSet<>();
        
        Set<String> queue = new HashSet<>();
        queue.add(start);
        visited.add(start);
        
        Map<String, Set<String>> parents = new HashMap<>();
        
        boolean found = false;
        while(!queue.isEmpty()) {
            
            Set<String> nextLevel = new HashSet<>();
            
            for(String current : queue) {
                if(current.equals(end)) {
                    found = true;
                    break;
                }
                
                char[] chars = current.toCharArray();
                for(int i = 0; i<len; i++){
                    char original = chars[i];
                    for(int c='a'; c<='z'; c++) {
                        chars[i] = (char)c;
                        String s = String.valueOf(chars);
                        if(!visited.contains(s) && dict.contains(s)) {
                            nextLevel.add(s);
                            visited.add(s);
                        }
                        if(nextLevel.contains(s) && dict.contains(s)){
                            if(!parents.containsKey(s))
                                parents.put(s, new HashSet<>());
                            parents.get(s).add(current);
                        }
                        chars[i] = original;
                    }
                }
                
            }
            
            queue = nextLevel;
        }
        
        if(found) {
            //dfs parents
            return dfs(end, start, parents);
        }else{
            return new ArrayList<>();
        }
        
    }
    
    private List<List<String>> dfs(String current, String end, Map<String, Set<String>> parents) {
        if(current.equals(end)) {
            List<List<String>> list = new ArrayList<>();
            List<String> item = new ArrayList<>();
            item.add(current);
            list.add(item);
            return list;
        }
        
        List<List<String>> list = new ArrayList<>();
        for(String p : parents.get(current)) {
            for(List<String> item : dfs(p, end, parents)) {
                item.add(current);
                list.add(item);
            }
        }
        return list;
    }