<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>

class Solution {
public:void dfs(string curr, vector<string> path, unordered_map<string, int>& mp, vector<vector<string>>& ans) {if (mp[curr] == 0) {reverse(path.begin(), path.end());ans.push_back(path);return;}int n = curr.size();string w = curr;for (int i = 0; i < n; i++) {char org = curr[i];for (char c = 'a'; c <= 'z'; c++) {if (c == org)continue;curr[i] = c;if (mp.count(curr) && mp[curr] == mp[w] - 1) {vector<string> new_path(path);new_path.push_back(curr);dfs(curr, new_path, mp, ans);}}curr[i] = org;}}
vector<vector<string>> findLadders(string startWord, string endWord,   vector<string>& wordList) {vector<vector<string>> ans;unordered_set<string> wordSet(wordList.begin(), wordList.end());if (!wordSet.count(endWord))return ans;
vector<vector<int>> paths;wordSet.insert(startWord);queue<string> q;unordered_map<string, int> mp;wordSet.erase(startWord);q.push(startWord);int lvl = 1, tar_lvl = INT_MAX;mp[startWord] = 0;while (!q.empty()) {int sz = q.size();for (int i = 0; i < sz; i++) {auto word = q.front();q.pop();
int n = word.size();for (int i = 0; i < n; i++) {char org = word[i];
for (char c = 'a'; c <= 'z'; c++) {if (c == org)continue;
word[i] = c;
if (word == endWord) {tar_lvl = lvl;mp[word] = lvl;continue;}
if (wordSet.count(word)) {mp[word] = lvl;q.emplace(word);wordSet.erase(word);}}word[i] = org;}}
if (tar_lvl == lvl) {break;}lvl++;}
if (tar_lvl != INT_MAX)dfs(endWord, vector<string>{endWord}, mp, ans);
return ans;}
};

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