LC 126. Word Ladder II

link

class Solution {
public:
    vector<vector<string>> findLadders(string beginWord, string endWord, vector<string>& wordList) {
        vector<vector<string>> res;
        unordered_set<string> words;
        for(auto s:wordList){
            words.insert(s);
        }
        if(words.find(endWord)==words.end()) return res;
        unordered_map<string, vector<vector<string>>> path;
        path[beginWord]={{beginWord}};
        queue<string> q;
        q.push(beginWord);
        while(!q.empty()){
            string cur=q.front();
            q.pop();
            if(cur==endWord) break;
            vector<string> nexts=findNext(cur,words);
            
            for(string next:nexts){
                if(path.find(next)==path.end()){
                    q.push(next);
                    path[next]=path[cur];
                    for(auto &v:path[next]){
                        v.push_back(next);
                    }
                }else if(path[cur][0].size()+1==path[next][0].size()){
                    auto tmp=path[cur];
                    for(auto &v:tmp){
                        v.push_back(next);
                        path[next].push_back(v);
                    }
                }
            }
        }
        if(path.find(endWord)==path.end()) return res;
        return path[endWord];
    }
    
    vector<string> findNext(string s, unordered_set<string> &words){
        vector<string> res;
        for(int i=0;i<s.size();++i){
            char bk=s[i];
            for(char c='a';c<='z';++c){
                if(c==bk) continue;
                s[i]=c;
                if(words.find(s)!=words.end()) res.push_back(s);
            }
            s[i]=bk;
        }
        return res;
    }
};