Word Ladder II
<p>A <strong>transformation sequence</strong> from word <code>beginWord</code> to word <code>endWord</code> using a dictionary <code>wordList</code> is a sequence of words <code>beginWord -&gt; s<sub>1</sub> -&gt; s<sub>2</sub> -&gt; ... -&gt; s<sub>k</sub></code> such that:</p>

<ul>
	<li>Every adjacent pair of words differs by a single letter.</li>
	<li>Every <code>s<sub>i</sub></code> for <code>1 &lt;= i &lt;= k</code> is in <code>wordList</code>. Note that <code>beginWord</code> does not need to be in <code>wordList</code>.</li>
	<li><code>s<sub>k</sub> == endWord</code></li>
</ul>

<p>Given two words, <code>beginWord</code> and <code>endWord</code>, and a dictionary <code>wordList</code>, return <em>all the <strong>shortest transformation sequences</strong> from</em> <code>beginWord</code> <em>to</em> <code>endWord</code><em>, or an empty list if no such sequence exists. Each sequence should be returned as a list of the words </em><code>[beginWord, s<sub>1</sub>, s<sub>2</sub>, ..., s<sub>k</sub>]</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> beginWord = &quot;hit&quot;, endWord = &quot;cog&quot;, wordList = [&quot;hot&quot;,&quot;dot&quot;,&quot;dog&quot;,&quot;lot&quot;,&quot;log&quot;,&quot;cog&quot;]
<strong>Output:</strong> [[&quot;hit&quot;,&quot;hot&quot;,&quot;dot&quot;,&quot;dog&quot;,&quot;cog&quot;],[&quot;hit&quot;,&quot;hot&quot;,&quot;lot&quot;,&quot;log&quot;,&quot;cog&quot;]]
<strong>Explanation:</strong>&nbsp;There are 2 shortest transformation sequences:
&quot;hit&quot; -&gt; &quot;hot&quot; -&gt; &quot;dot&quot; -&gt; &quot;dog&quot; -&gt; &quot;cog&quot;
&quot;hit&quot; -&gt; &quot;hot&quot; -&gt; &quot;lot&quot; -&gt; &quot;log&quot; -&gt; &quot;cog&quot;
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> beginWord = &quot;hit&quot;, endWord = &quot;cog&quot;, wordList = [&quot;hot&quot;,&quot;dot&quot;,&quot;dog&quot;,&quot;lot&quot;,&quot;log&quot;]
<strong>Output:</strong> []
<strong>Explanation:</strong> The endWord &quot;cog&quot; is not in wordList, therefore there is no valid transformation sequence.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= beginWord.length &lt;= 5</code></li>
	<li><code>endWord.length == beginWord.length</code></li>
	<li><code>1 &lt;= wordList.length &lt;= 500</code></li>
	<li><code>wordList[i].length == beginWord.length</code></li>
	<li><code>beginWord</code>, <code>endWord</code>, and <code>wordList[i]</code> consist of lowercase English letters.</li>
	<li><code>beginWord != endWord</code></li>
	<li>All the words in <code>wordList</code> are <strong>unique</strong>.</li>
	<li>The <strong>sum</strong> of all shortest transformation sequences does not exceed <code>10<sup>5</sup></code>.</li>
</ul>
Breadth-First Search; Hash Table; String; Backtracking
class Solution {
    private List<List<String>> ans;
    private Map<String, Set<String>> prev;

    public List<List<String>> findLadders(String beginWord, String endWord, List<String> wordList) {
        ans = new ArrayList<>();
        Set<String> words = new HashSet<>(wordList);
        if (!words.contains(endWord)) {
            return ans;
        }
        words.remove(beginWord);
        Map<String, Integer> dist = new HashMap<>();
        dist.put(beginWord, 0);
        prev = new HashMap<>();
        Queue<String> q = new ArrayDeque<>();
        q.offer(beginWord);
        boolean found = false;
        int step = 0;
        while (!q.isEmpty() && !found) {
            ++step;
            for (int i = q.size(); i > 0; --i) {
                String p = q.poll();
                char[] chars = p.toCharArray();
                for (int j = 0; j < chars.length; ++j) {
                    char ch = chars[j];
                    for (char k = 'a'; k <= 'z'; ++k) {
                        chars[j] = k;
                        String t = new String(chars);
                        if (dist.getOrDefault(t, 0) == step) {
                            prev.get(t).add(p);
                        }
                        if (!words.contains(t)) {
                            continue;
                        }
                        prev.computeIfAbsent(t, key -> new HashSet<>()).add(p);
                        words.remove(t);
                        q.offer(t);
                        dist.put(t, step);
                        if (endWord.equals(t)) {
                            found = true;
                        }
                    }
                    chars[j] = ch;
                }
            }
        }
        if (found) {
            Deque<String> path = new ArrayDeque<>();
            path.add(endWord);
            dfs(path, beginWord, endWord);
        }
        return ans;
    }

    private void dfs(Deque<String> path, String beginWord, String cur) {
        if (cur.equals(beginWord)) {
            ans.add(new ArrayList<>(path));
            return;
        }
        for (String precursor : prev.get(cur)) {
            path.addFirst(precursor);
            dfs(path, beginWord, precursor);
            path.removeFirst();
        }
    }
}