127. Word ladder

把这道题想象成bfs，每次生成下次需要访问的一层，然后交替。

然后可以从两头bfs，像挖隧道一样，两边那边更少就从那边走，直到中间有汇合的点，或者两边都挖不下去了

 

    public int ladderLength(String beginWord, String endWord, Set<String> wordList) {
        if(beginWord == null || endWord == null || endWord.length() != beginWord.length() || wordList == null || wordList.size() == 0) {
            return 0;
        }
        int len = beginWord.length();
        Set<String> beginSet = new HashSet<String>();
        Set<String> endSet = new HashSet<String>();
        Set<String> visited = new HashSet<String>();
        beginSet.add(beginWord);
        endSet.add(endWord);
        visited.add(beginWord);
        int dist = 1;
        while(!beginSet.isEmpty() && !endSet.isEmpty()) {
            if(beginSet.size() > endSet.size()) {
                Set<String> temp = beginSet;
                beginSet = endSet;
                endSet = temp;
            }
            Set<String> toAdd = new HashSet<String>();
            for(String each: beginSet) {
                char[] chs = each.toCharArray();
                for(int i = 0; i < len; i++) {
                    char ch = chs[i];
                    for(char c = 'a'; c <= 'z'; c++) {
                        chs[i] = c;
                        String tempWord = new String(chs);
                        if(endSet.contains(tempWord)) {
                            return ++dist;
                        }
                        if(wordList.contains(tempWord) && !visited.contains(tempWord)) {
                            toAdd.add(tempWord);
                            wordList.remove(tempWord);
                            visited.add(tempWord);
                        }
                    }
                    chs[i] = ch;
                }
            }
            dist++;
            beginSet = toAdd;
        }
        return 0;
    }