<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:
    bool isConnected(string s,string t){
        int c=0;
        for(int i=0;i<s.length();i++)
            c+=(s[i]!=t[i]);
        return c==1;
    }

    vector<vector<string>> findLadders(string beginWord, string endWord, vector<string>& wordList) {
        vector<vector<string>> ans;        
        vector<vector<string>> nodes;   
        unordered_set<string> dict(wordList.begin(),wordList.end());
        
        if (!dict.count(endWord)) return ans;
        dict.erase(beginWord);
        
        
        bool reached = false;
        nodes.push_back({beginWord});
        
        while (dict.size() && !reached) {                        
            vector<string> last = nodes.back();
            vector<string> curr;
                        
            for (int i = 0; i < last.size() && !reached; i++) {
                unordered_set<string> visited;
                string from = last[i];                                                
                for (auto& to : dict) {                    
                    if (visited.count(to)) continue;
                    if (!isConnected(from, to)) continue;                                        
                    if (to == endWord) {                        
                        reached = true; 
                        break;
                    }
                    curr.push_back(to);   
                    visited.insert(to);                    
                }           
                for (auto& visited : visited) {                
                    dict.erase(visited);
                }
            }
            if (reached) break;
            if (!curr.size()) break;
            nodes.push_back(curr);            
        }
        if (reached == false) return ans;
        ans.push_back({endWord});          
        for (int level = nodes.size() - 1; level >= 0; level--) {                        
            int alen = ans.size();
            while (alen) {
                vector<string> path = ans.back();
                ans.pop_back();
                string from = path.front();                
                for (string &to : nodes[level]) {                    
                    if (!isConnected(from, to)) continue;
                                        
                    vector<string> newpath = path;
                    newpath.insert(newpath.begin(), to);
                    ans.insert(ans.begin(), newpath);
                }    
                alen--;
            }  //Code With Avinash           
        }
        return ans;
    }
};
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