class Solution {
    public List<List<String>> findLadders(String beginWord, String endWord, List<String> wordList) {
        Set<String> words = new HashSet<>(wordList);
        List<List<String>> ans = new ArrayList<>();
        if (!words.contains(endWord)) {
            return ans;
        }

        Map<String, List<String>> adjWords = new HashMap<>();
        Map<String, Integer> dist = new HashMap<>();
        bfs(beginWord, adjWords, dist, words);
        dfs(endWord, beginWord, adjWords, dist, new ArrayList<>(), ans);

        return ans;
    }

    public void bfs(String beginWord, Map<String, List<String>> adjWords, Map<String, Integer> dist, Set<String> words) {
        Queue<String> queue = new LinkedList<>();
        queue.offer(beginWord);
        dist.put(beginWord, 0);
        for (String word : words) {
            adjWords.put(word, new ArrayList<>());
        }

        while (!queue.isEmpty()) {
            String cur = queue.poll();
            List<String> neighbors = neighbors(cur, words);
            for (String next : neighbors) {
                adjWords.get(next).add(cur);
                if (!dist.containsKey(next)) {
                    dist.put(next, dist.get(cur) + 1);
                    queue.offer(next);
                }
            }
        }
    } 

    public void dfs(String curWord, String beginWord,Map<String, List<String>> adjWords, Map<String, Integer> dist, List<String> path, List<List<String>> ans) {
        path.add(curWord);
        if (curWord.equals(beginWord)) {
            Collections.reverse(path);
            ans.add(new ArrayList<>(path));
            Collections.reverse(path);
        } else {
            for (String next : adjWords.get(curWord)) {
                if (dist.containsKey(next) && dist.get(curWord) == dist.get(next) + 1) {
                    dfs(next, beginWord, adjWords, dist, path, ans);
                }
            }
        }

        path.remove(path.size() - 1);
    }

    public List<String> neighbors(String word, Set<String> words) {
        List<String> ans = new ArrayList<>();
        char[] sc = word.toCharArray();
        for (int i = 0; i < sc.length; i++) {
            char cur = sc[i];
            for (char c = 'a'; c <= 'z'; c++) {
                sc[i] = c;
                String temp = new String(sc);
                if (words.contains(temp)) {
                    ans.add(temp);
                }
            }
            sc[i] = cur;
        }

        return ans;
    }
}

리트코드

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