<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>

int getDistance(string s1, string s2) {
    if (s1.size() != s2.size()) {
        return -1;
    }
    int res = 0;
    for (int i = 0; i < s1.size(); i++) {
        res += !!(s1[i] != s2[i]);
    }
    return res;
}

class Solution {
public:
    int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
        wordList.push_back(beginWord);
        vector<unordered_set<int>> g(wordList.size());
        vector<int> distance(wordList.size());
        int endWordId = -1;

        for (int k = 0; k < beginWord.size(); k++) {
            vector<pair<string, int>> v;
            for(int j = 0; j < wordList.size(); j++) {
                v.push_back({wordList[j].substr(0, k) + wordList[j].substr(k + 1, beginWord.size() - k - 1), j});
            }
            sort(v.begin(), v.end());
            int left = 0;
            for (int right = left + 1; right <= v.size(); right++) {
                if (right == v.size() || v[right].first != v[left].first) {
                    for (int i = left; i < right; i++) {
                        for (int j = left + 1; j < right; j++) {
                            if (v[i].second != v[j].second) {
                                g[v[i].second].insert(v[j].second);
                                g[v[j].second].insert(v[i].second);
                            }
                        }
                    }
                    left = right;
                }
            }
        }

        // for (int i = 0; i < wordList.size(); i++) {
        //     cout << wordList[i] << ": ";
        //     for (auto it : g[i]) {
        //         cout << wordList[it] << " ";
        //     }
        //     cout << endl;
        // }


        for (int i = 0; i < wordList.size(); i++) {
            if (endWord == wordList[i]) {
                endWordId = i;
            }
        }

        if (endWordId == -1) {
            return 0;
        }


        list<int> q;
        q.push_back(wordList.size() - 1);
        distance[wordList.size() - 1] = 1;

        while(q.size()) {
            int v = q.front();
            q.pop_front();
            for (auto u : g[v]) {
                if (distance[u] == 0) {
                    q.push_back(u);
                    distance[u] = distance[v] + 1;
                }
            }
        }
        // for (int i = 0; i < distance.size(); i++) {
        //     cout << wordList[i] << " " << distance[i] << endl;
        // }
        return distance[endWordId];
    }
};
50575