127.Word Ladder

class Solution {
public:
    int ladderLength(string beginWord, string endWord, unordered_set<string>& wordDict) {
        unordered_set<string>begset,endset,*set1,*set2;
        begset.insert(beginWord);
        endset.insert(endWord);
        int res=2, wordLen=beginWord.size();
        while(!begset.empty()&&!endset.empty()){
            if(begset.size()<=endset.size()){
                set1 = &begset;
                set2 = &endset;
            }
            else{
                set1=&endset;
                set2=&begset;
            }
            
            unordered_set<string> item;
            for(auto itr=set1->begin();itr!=set1->end();itr++){
                string str=*itr;
                for(int wordIndex=0; wordIndex<wordLen; wordIndex++){
                    char orich=str[wordIndex];
                    for(int ch='a';ch<='z';ch++){
                        str[wordIndex]=ch;
                        if(set2->find(str)!=set2->end())
                            return res;
                        else{
                            if(wordDict.find(str)!=wordDict.end()){
                                item.insert(str);
                                wordDict.erase(str);
                            }
                        }
                    }
                    str[wordIndex]=orich;
                }
            }
            swap(*set1,item);
            res++;
        }
        return 0;
    }
};