127. Word Ladder

class Solution {
    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        Set<String> dict=new HashSet<String>(wordList);
        if(!dict.contains(endWord))
            return 0;
        Set<String> set1=new HashSet<String>(Arrays.asList(beginWord));
        Set<String> set2=new HashSet<String>(Arrays.asList(endWord));
        return ladderLength(set1, set2, dict, 1);
    }
    private int ladderLength(Set<String> set1, Set<String> set2, Set<String> dict, int level) {
        if(set1.size()==0)
            return 0;
        if(set1.size()>set2.size())
            return ladderLength(set2, set1, dict, level);
        dict.removeAll(set1);
        dict.removeAll(set2);
        Set<String> set3=new HashSet<String>();
        for(String w:set1)
            for(int i=0;i<w.length();i++)
            {
                char[] arr=w.toCharArray();
                for(char ch='a';ch<='z';ch++)
                {
                    arr[i]=ch;
                    String word=new String(arr);
                    if(set2.contains(word))
                        return level+1;
                    if(dict.contains(word))
                        set3.add(word);
                }
            }
        return ladderLength(set2, set3, dict, level+1);
    }
}