leetcode--Word Ladder

Given two words (beginWord and endWord), and a dictionary, find the length of shortest transformation sequence from
beginWord to endWord, such that:

Only one letter can be changed at a time
Each intermediate word must exist in the dictionary
For example,

Given:
start =

"hit"

end =

"cog"

dict =

["hot","dot","dog","lot","log"]

As one shortest transformation is

"hit" -> "hot" -> "dot" -> "dog" -> "cog"

,

return its length

5

.

Note:

Return 0 if there is no such transformation sequence.
All words have the same length.
All words contain only lowercase alphabetic characters.

public class Solution {
public int ladderLength(String beginWord, String endWord, Set<String> wordDict) {
class WordNode{
String word;
int step;
public WordNode(String word, int step) {
super();
this.word = word;
this.step = step;
}
}
Queue<WordNode> queue = new LinkedList<WordNode>();
queue.add(new WordNode(beginWord,1));
wordDict.add(endWord);
while(!queue.isEmpty()){
WordNode cur = queue.poll();
if(cur.word.equals(endWord)){
return cur.step;
}else{
char[] arr = cur.word.toCharArray();
for(int i=0;i<arr.length;i++){
for(int c='a';c<='z';c++){
char temp = arr[i];
if(c!=temp){
arr[i] = (char) c;
}
String str = new String(arr);
if(wordDict.contains(str)){
queue.add(new WordNode(str, cur.step+1));
wordDict.remove(str);
}
arr[i] = temp;
}
}
}
}
return 0;
}
}