127.Word Ladder

1-Queue + Set

https://leetcode.com/problems/word-ladder/discuss/40707/Easy-76ms-C++-Solution-using-BFS

class Solution {
    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        Set<String> dict = new HashSet<>(wordList);
        Set<String> visited = new HashSet<>();
        Deque<String> queue = new ArrayDeque<>();
        queue.offerLast(beginWord);
        for(int len = 1; !queue.isEmpty(); len++){
            for(int i = queue.size(); i > 0; i--){
                String cur = queue.pollFirst();
                if(cur.equals(endWord))
                    return len;
                for(int j = 0; j < cur.length(); j++){
                    char[] ch = cur.toCharArray();
                    for(char c = 'a'; c <= 'z'; c++){
                        if(c == cur.charAt(j))
                            continue;
                        ch[j] = c;
                        String newString = new String(ch);
                        if(dict.contains(newString) && visited.add(newString))
                            queue.offerLast(newString);
                    }
                }
            }
        }
        return 0;
    }
}