vector<string>

class Solution{
    public:
    int n;
    vector<int> v;
    vector<string> vs_;
    unordered_map<char, vector<int>> um;
    bool Circle(vector<string> vs){
        int save=0;
        for(int i=0;i<vs.size();i++){
            if(vs[i]!="") vs[save++]=vs[i];
        }
        vs.resize(save);
        vs_=vs;
        um.clear();
        n=vs.size();
        for(int i=0;i<n;i++){
            um[vs[i][0]].push_back(i);
        }

        v.assign(n, 0);
        //int start=0, cnt=1;;
        //int index=0;
        return dfs(0, 0);
        //return v[index].back()==v[start][0];
    }

    bool dfs(int now, int i){
        if(!um.count(vs_[i].back())) return 0;
        if(now>=n && i==0) return 1;
        for(auto e: um[vs_[i].back()]){
            int index=e;
            if(v[index]) continue;
            v[index]=1;
            if(dfs(now+1, index)) return 1;
            v[index]=0;
        }
        return 0;
    }
} S;