Word Ladder II

本身寫的一個算法，用node來代替存儲每一個可行點的歷史串。可是LC仍是過不了，由於超時：

//To avoid memory issues, use new node for it

   private  class WLNode {
        public String str;
        public WLNode parent;
        public WLNode(String str, WLNode parent){
            this.str = str;
            this.parent = parent;
        }
    }

    private void  findRlt(WLNode node, List<List<String>> rlt){
//        print("find a solution ");
        List<String> item = new ArrayList<String>();
        WLNode cur = node;
        while(cur != null){
            item.add(0, cur.str);
            cur = cur.parent;
        }
        rlt.add(item);
    }
    public List<List<String>> findLadders(String beginWord, String endWord,Set<String> wordList) {
        //key is that each node needs a level
        List<List<String>> rlt = new ArrayList<List<String>>();
        wordList.add(beginWord); wordList.add(endWord);
        //map to determine levels
        Map<String, Integer> levelMap = new HashMap<String, Integer>();
        for(String temp : wordList){
            levelMap.put(temp, 1);
        }
        Queue<WLNode> que = new LinkedList<WLNode>();
        que.add(new WLNode(beginWord, null));
        while(!que.isEmpty()){
            WLNode node = que.remove();
            String cur = node.str;
            int curLevel = levelMap.get(cur);
            // query
            for(int i = 0; i < cur.length(); i ++){
                for(char x = 'a'; x <= 'z'; x ++){
                    if(cur.charAt(i) == x){
                        continue;
                    }
                    String next = cur.substring(0, i) + x +  cur.substring(i + 1);

                    if(wordList.contains(next)){
                        if(next.equals(endWord) && curLevel + 1 > levelMap.get(endWord) && levelMap.get(endWord) != 1){
//                            print("Found solution!!");
                            break;
                        }
                        if(levelMap.get(next) == 1 || curLevel + 1 <= levelMap.get(next)){
                            WLNode nodeToAdd = new WLNode(next, node);
                            levelMap.put(next, curLevel + 1);
                            que.add(nodeToAdd);
                            if(next.equals(endWord)){
                                findRlt(nodeToAdd, rlt);
                            }
                        }
                    }
                }
            }
        }
        return rlt;
    }

看看標準答案： 

 果真是應該先BFS而後DFS，這樣一來能夠不用index根本不會遇到的路徑，二來從後往前走能夠避免不須要的路。