Middle_119_127_Word Ladder.java

public class Solution {

public int ladderLength(String beginWord, String endWord, Set<String> wordList) {
    Set<String> beginSet = new HashSet<String>(), endSet = new HashSet<String>();

    int len = 1;
    int strLen = beginWord.length();
    HashSet<String> visited = new HashSet<String>();

    beginSet.add(beginWord);
    endSet.add(endWord);
    while (!beginSet.isEmpty() && !endSet.isEmpty()) {
        if (beginSet.size() > endSet.size()) {
            Set<String> set = beginSet;
            beginSet = endSet;
            endSet = set;
        }

        Set<String> temp = new HashSet<String>();
        for (String word : beginSet) {
            char[] chs = word.toCharArray();

            for (int i = 0; i < chs.length; i++) {
                for (char c = 'a'; c <= 'z'; c++) {
                    char old = chs[i];
                    chs[i] = c;
                    String target = String.valueOf(chs);

                    if (endSet.contains(target)) {
                        return len + 1;
                    }

                    if (!visited.contains(target) && wordList.contains(target)) {
                        temp.add(target);
                        visited.add(target);
                    }
                    chs[i] = old;
                }
            }
        }

        beginSet = temp;
        len++;
    }

    return 0;
}
}