<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) {
        if (beginWord.size() != endWord.size()) return 0;

        unordered_map<string,vector<int>> hashMap;
        bool isEndWord = false;
        for (int j = 0; j < wordList.size(); j++) {
            string word = wordList[j];
            if (word == endWord) 
                isEndWord = true;
            for (int i = 0; i < word.size(); i++) {
                char tmp = word[i];
                word[i] = '*';
                hashMap[word].push_back(j);
                word[i] = tmp;
            }
        }

        if (!isEndWord) return 0;

        auto _func = [](unordered_map<string,vector<int>>& hashMap, vector<string>& wordList, queue<pair<string,int>> &q, unordered_map<string, int> &beginWords, unordered_map<string, int> &endWords) {
            for (int j = q.size(); j > 0; j--) {
                auto word = q.front();
                q.pop();

                for (int i = 0; i < word.first.size(); i++) {
                    char tmp = word.first[i];
                    word.first[i] = '*';
                    
                    auto it = hashMap.find(word.first);
                    if (it != hashMap.end()) {
                        for (auto idx: it->second) {
                            if (endWords.find(wordList[idx]) != endWords.end()) {
                                return word.second + endWords[wordList[idx]];
                            }
                            if (beginWords.find(wordList[idx]) == beginWords.end()) {
                                q.push({wordList[idx], word.second+1});
                                beginWords[wordList[idx]] = word.second+1;
                            }
                        }
                    }

                    word.first[i] = tmp;
                }
            }

            return -1;
        };

        queue<pair<string,int>> qTopDown;
        queue<pair<string,int>> qBottomUp;
        qTopDown.push({beginWord, 1});
        qBottomUp.push({endWord, 1});

        unordered_map<string,int> beginWords;
        unordered_map<string,int> endWords;
        beginWords[beginWord] = 1;
        endWords[endWord] = 1;

        int ans = -1;
        while(!qTopDown.empty() && !qBottomUp.empty()) {
            
            if (qTopDown.size() <= qBottomUp.size())
                ans = _func(hashMap, wordList, qTopDown, beginWords, endWords);
            else
                ans = _func(hashMap, wordList, qBottomUp, endWords, beginWords);

            if (ans > -1) {
                return ans;
            }
        }
        return 0;
    }
};
40175