LeetCode in Net

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

using System.Collections.Generic;

public class Solution {
    public int LadderLength(string beginWord, string endWord, IList<string> wordDict) {
        var beginSet = new HashSet<string>();
        var endSet = new HashSet<string>();
        var wordSet = new HashSet<string>(wordDict);
        var visited = new HashSet<string>();
        if (!wordDict.Contains(endWord)) {
            return 0;
        }
        int len = 1;
        int strLen = beginWord.Length;
        beginSet.Add(beginWord);
        endSet.Add(endWord);
        while (beginSet.Count > 0 && endSet.Count > 0) {
            if (beginSet.Count > endSet.Count) {
                var temp = beginSet;
                beginSet = endSet;
                endSet = temp;
            }
            var tempSet = new HashSet<string>();
            foreach (var s in 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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