LeetCode-in-Java

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 -> ... -> sk such that:

• Every adjacent pair of words differs by a single letter.
• Every si for 1 <= i <= k is in wordList. Note that beginWord does not need to be in wordList.
• sk == 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.

Solution

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

public class Solution {
    public int ladderLength(String beginWord, String endWord, List<String> wordDict) {
        Set<String> beginSet = new HashSet<>();
        Set<String> endSet = new HashSet<>();
        Set<String> wordSet = new HashSet<>(wordDict);
        Set<String> 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<String> temp = beginSet;
                beginSet = endSet;
                endSet = temp;
            }
            Set<String> 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;
    }
}

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