<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 travel(vector<vector<string>>& vvs, vector<string>& vs,vector<string>& wl, vector<vector<int>>& last, int cur){
      if(last[cur][0]==-1){
        vs.push_back(wl[cur]);
        vvs.push_back(vs);
        vs.pop_back();
        return;
      }
      vs.push_back(wl[cur]);
      for(int i : last[cur]){
        travel(vvs,vs,wl,last,i);
      }
      vs.pop_back();
    }
    vector<vector<string>> findLadders(string bw, string ew, vector<string>& wl) {
        int cur=find(wl.begin(),wl.end(),bw)-wl.begin();
        if(cur==wl.size()){
          wl.push_back(bw);
        }else{
          swap(wl[cur],wl.back());
        }

        vector<vector<int>> last(wl.size());
        vector<int> t(wl.size(),0);
        
        cur=find(wl.begin(),wl.end(),ew)-wl.begin();
        if(cur==wl.size())
          return {};
        last[cur].push_back(-1);
        t[cur]=1;

        vector<int> q[2];
        int r=0,s=1;
        q[r].push_back(cur);
        int time=2;
        int count;
        while(last.back().empty()&&!q[r].empty()){
          //
          // cout<<"q ";
          // for(int i : q[r])
          //   cout<<wl[i]<<" ";
          // cout<<endl;
          //
          q[s].clear();
          for(int i : q[r]){
            for(int j=0;j<wl.size();j++){
              if(t[j]==0||t[j]==time){
                count=0;
                for(int k=0;k<bw.size();k++){
                  count+=(wl[i][k]!=wl[j][k]);
                }
                if(count==1){
                  last[j].push_back(i);
                  if(t[j]==0)
                    q[s].push_back(j);
                  t[j]=time;
                }
              }
            }
          }
          swap(r,s);
          time++;
        }
        if(q[r].empty())
          return {};
        //
        // for(int i=0;i<last.size();i++){
        //   cout<<wl[i]<<" ";
        //   for(auto j:last[i]){
        //     cout<<j<<" ";
        //   }
        //   cout<<endl;
        // }
        //
        vector<vector<string>> vvs;
        vector<string> vs;
        travel(vvs,vs,wl,last,wl.size()-1);
        return vvs;
    }
};
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