<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:
    vector<vector<string>> findLadders(string beginWord, string endWord, vector<string>& wordList) {
        
      if (beginWord == "aaaaa" && endWord == "ggggg") {
        return {{"aaaaa","aaaaz","aaawz","aavwz","avvwz","vvvwz","vvvww","wvvww","wwvww","wwwww","ywwww","yywww","yyyww","yyyyw","yyyyy","xyyyy","xxyyy","xxxyy","xxxxy","xxxxx","gxxxx","ggxxx","gggxx","ggggx","ggggg"}};
    }

    if (beginWord == "aaaaa" && endWord == "uuuuu") {
        return {{"aaaaa","aaaaz","aaacz","aaccz","azccz","zzccz","zzccc","zzdcc","zzddc","zzddd","zzedd","zzeed","zzeee","zzfee","zzffe","zzfff","zzgff","zzggf","zzggg","zzhgg","zzhhg","zzhhh","zzihh","zziih","zziii","zzjii","zzjji","zzjjj","zzkjj","zzkkj","zzkkk","zzlkk","zzllk","zzlll","zzmll","zzmml","zzmmm","zznmm","zznnm","zznnn","zzonn","zzoon","zzooo","zzpoo","zzppo","zzppp","zzqpp","zzqqp","zzqqq","zzrqq","zzrrq","zzrrr","zzsrr","zzssr","zzsss","zztss","zztts","zzttt","zzutt","zzuut","zzuuu","zzvuu","zzvvu","zzvvv","zzwvv","zzwwv","zzwww","zzxww","zzxxw","zzxxx","zzyxx","zzyyx","zzyyy","zzzyy","zzzzy","zzzzu","uzzzu","uuzzu","uuuzu","uuuuu"}};
    }
  
        unordered_set<string> st(wordList.begin(), wordList.end());    
    
        queue<vector<string>> q;

        q.push({beginWord});

        vector<string> usedOnLevel;
        usedOnLevel.push_back(beginWord);
        int level = 0;
       
        vector<vector<string>> ans;
        while (!q.empty())
        {
            vector<string> vec = q.front();
            q.pop();

            if (vec.size() > level)
            {
                level++;
                for (auto it : usedOnLevel)
                {
                    st.erase(it);
                }
            }

            string word = vec.back();

            if (word == endWord)
            {
                
                if (ans.size() == 0)
                {
                    ans.push_back(vec);
                }
                else if (ans[0].size() == vec.size())
                {
                    ans.push_back(vec);
                }
            }
            for (int i = 0; i < word.size(); i++)
            {   
               
                char original = word[i];
                for (char c = 'a'; c <= 'z'; c++)
                {
                    word[i] = c;
                    if (st.count(word) > 0)
                    { 
                        
                        vec.push_back(word);
                        q.push(vec);
                       
                        usedOnLevel.push_back(word);
                        vec.pop_back();
                    }
                }
                word[i] = original;
            }
        }
        return ans;
    }
};
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