<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>the <strong>number of words</strong> in the <strong>shortest transformation sequence</strong> from</em> <code>beginWord</code> <em>to</em> <code>endWord</code><em>, or </em><code>0</code><em> if no such sequence exists.</em></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> 5
<strong>Explanation:</strong> One shortest transformation sequence is &quot;hit&quot; -&gt; &quot;hot&quot; -&gt; &quot;dot&quot; -&gt; &quot;dog&quot; -&gt; cog&quot;, which is 5 words long.
</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> 0
<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;= 10</code></li>
	<li><code>endWord.length == beginWord.length</code></li>
	<li><code>1 &lt;= wordList.length &lt;= 5000</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>
</ul>

class Solution {
public:
    int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
        ios::sync_with_stdio(0);
        cin.tie(0);
        unordered_set<string> visited;
        unordered_map<string,vector<string>> wordMap;
        unordered_set<string> wordSet;
        queue<string> q;
        int n = beginWord.length();
        wordList.push_back(beginWord);
        for(string& s: wordList)
            wordSet.insert(s);
        for(auto& s: wordList){
            vector<string> branches;
            for(int i=0;i<n;i++){
                string nextWord = s;
                for(int j=0;j<26;j++){
                    nextWord[i] = j + 'a';
                    if(nextWord == s)
                        continue;
                    if(wordSet.find(nextWord)!=wordSet.end()){
                        branches.push_back(nextWord);
                        // cout<<s<<"-"<<nextWord<<" ";
                    }
                }
            }
            wordMap[s] = branches;
            // cout<<s<<endl;
        }
        q.push(beginWord);
        int level=1;
        while(!q.empty()){
            queue<string> q1;
            while(!q.empty()){
                string curr = q.front();
                // cout<<curr<<" ";
                q.pop();
                vector<string> branches = wordMap[curr];
                // cout<<branches.size()<<endl;
                for(auto& next: branches){
                    // cout<<next<<" ";
                    if(visited.find(next) != visited.end())
                        continue;
                    if(next == endWord)
                        return level+1;
                    q1.push(next);
                    visited.insert(next);
                }
            }
            q=q1;
            level++;
        }
        return 0;
    }
};
57075