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Java-based LeetCode algorithm problem solutions, regularly updated
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package g0101_0200.s0127_word_ladder;

// #Hard #Top_Interview_Questions #String #Hash_Table #Breadth_First_Search
// #Graph_Theory_I_Day_12_Breadth_First_Search
// #2022_06_23_Time_37_ms_(94.58%)_Space_54.1_MB_(66.08%)

import java.util.HashSet;
import java.util.List;
import java.util.Set;

/**
 * 127 - Word Ladder\.
 *
 * Hard
 *
 * A **transformation sequence** from word `beginWord` to word `endWord` using a dictionary `wordList` is a sequence of words beginWord -> s₁ -> s₂ -> ... -> s_k such that:
 *
 * *   Every adjacent pair of words differs by a single letter.
 * *   Every s_i for `1 <= i <= k` is in `wordList`. Note that `beginWord` does not need to be in `wordList`.
 * *   s_k == endWord
 *
 * Given two words, `beginWord` and `endWord`, and a dictionary `wordList`, return _the **number of words** in the **shortest transformation sequence** from_ `beginWord` _to_ `endWord`_, or_ `0` _if no such sequence exists._
 *
 * **Example 1:**
 *
 * **Input:** beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"]
 *
 * **Output:** 5
 *
 * **Explanation:** One shortest transformation sequence is "hit" -> "hot" -> "dot" -> "dog" -> cog", which is 5 words long. 
 *
 * **Example 2:**
 *
 * **Input:** beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log"]
 *
 * **Output:** 0
 *
 * **Explanation:** The endWord "cog" is not in wordList, therefore there is no valid transformation sequence. 
 *
 * **Constraints:**
 *
 * *   `1 <= beginWord.length <= 10`
 * *   `endWord.length == beginWord.length`
 * *   `1 <= wordList.length <= 5000`
 * *   `wordList[i].length == beginWord.length`
 * *   `beginWord`, `endWord`, and `wordList[i]` consist of lowercase English letters.
 * *   `beginWord != endWord`
 * *   All the words in `wordList` are **unique**.
**/
public class Solution {
    public int ladderLength(String beginWord, String endWord, List wordDict) {
        Set beginSet = new HashSet<>();
        Set endSet = new HashSet<>();
        Set wordSet = new HashSet<>(wordDict);
        Set visited = new HashSet<>();
        if (!wordDict.contains(endWord)) {
            return 0;
        }
        int len = 1;
        int strLen = beginWord.length();
        beginSet.add(beginWord);
        endSet.add(endWord);
        while (!beginSet.isEmpty() && !endSet.isEmpty()) {
            if (beginSet.size() > endSet.size()) {
                Set temp = beginSet;
                beginSet = endSet;
                endSet = temp;
            }
            Set tempSet = new HashSet<>();
            for (String s : beginSet) {
                char[] chars = s.toCharArray();
                for (int i = 0; i < strLen; i++) {
                    char old = chars[i];
                    for (char j = 'a'; j <= 'z'; j++) {
                        chars[i] = j;
                        String temp = new String(chars);
                        if (endSet.contains(temp)) {
                            return len + 1;
                        }
                        if (!visited.contains(temp) && wordSet.contains(temp)) {
                            tempSet.add(temp);
                            visited.add(temp);
                        }
                    }
                    chars[i] = old;
                }
            }
            beginSet = tempSet;
            len++;
        }
        return 0;
    }
}