127. 单词接龙

和433.最小基因变化这道题一样的解法。
https://blog.csdn.net/qq_43606119/article/details/135538247

class Solution {
    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        Set<String> cnt = new HashSet<>();
        for (int i = 0; i < wordList.size(); ++i) {
            cnt.add(wordList.get(i));
        }
        if (!cnt.contains(endWord)) return 0;
        Set<String> visited = new HashSet<>();
        Queue<String> q = new ArrayDeque<>();
        q.offer(beginWord);
        visited.add(beginWord);
        int step = 1;
        while (!q.isEmpty()) {
            int size = q.size();
            for (int i = 0; i < size; ++i) {
                String curr = q.poll();
                for (int u = 0; u < curr.length(); ++u) {
                    for (int v = 0; v < 26; ++v) {
                        if (curr.charAt(u) != 'a' + v) {
                            StringBuffer sb = new StringBuffer(curr);
                            sb.setCharAt(u, (char)('a' + v));
                            String next = sb.toString();
                            if (!visited.contains(next) && cnt.contains(next)) {
                                if (next.equals(endWord)) return step + 1;
                                q.offer(next);
                                visited.add(next);
                            }
                        }
                    }
                }
            }
            ++step;
        }
        return 0;
    }
}

改进：

在本题中可以使用虚拟节点将各个单词进行连接，例如hit可以有三个虚拟节点hi*、h*t、*it，将其和hit进行连接，所有单词都如此处理，优化建图。

注意因为添加了虚拟节点，所以我们得到的距离为实际最短路径长度的两倍。同时我们并未计算起点对答案的贡献，所以我们应当返回距离的一半再加一的结果。

class Solution {
    Map<String, Integer> wordID = new HashMap<>();
    List<List<Integer>> edge = new ArrayList<>();
    int nodeNum = 0;
    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        for (String word : wordList) {
            addEdge(word);
        }
        addEdge(beginWord);
        if (!wordID.containsKey(endWord)) {
            return 0;
        }
        // 初始化dis数组，表示从beginWord到每个单词的最短转变路径长度，初始值为Integer.MAX_VALUE。
        int[] dis = new int[nodeNum];
        Arrays.fill(dis, Integer.MAX_VALUE);
        int beginID = wordID.get(beginWord), endID = wordID.get(endWord);
        dis[beginID] = 0;
        Queue<Integer> q = new LinkedList<>();
        q.offer(beginID);
        while (!q.isEmpty()) {
            int curr = q.poll();
            if (curr == endID) return dis[curr] / 2 + 1;
            for (int e : edge.get(curr)) {
                if (dis[e] == Integer.MAX_VALUE) {
                    dis[e] = dis[curr] + 1;
                    q.offer(e);
                }
            }
        }
        return 0;
    }

    private void addEdge(String word) {
        addWord(word);
        int id1 = wordID.get(word);
        char[] array = word.toCharArray();
        for (int i = 0; i < array.length; ++i) {
            char temp = array[i];
            array[i] = '*';
            String newWord = new String(array);
            addWord(newWord);
            int id2 = wordID.get(newWord);
            edge.get(id1).add(id2);
            edge.get(id2).add(id1);
            array[i] = temp;
        }
    }

    private void addWord(String word) {
        if (!wordID.containsKey(word)) {
            wordID.put(word, nodeNum++);
            edge.add(new ArrayList<Integer>());
        }
    }
}