枚举前缀哈希O1判断回文

int t;
    t =1;
    f[0]=1;
    for(int i=1;i<N;i++){
        f[i] = f[i-1]*ps;
    }
    while(t--){
        string s;
        cin >> s;
        if(s.size()==1){
            cout << s << endl;
            return 0;
        }
        ull p = 0,q = 0;
        int now = 0;
        for(int i=0;i<s.size();i++){
            p = p*ps + (s[i]-'a');
            q = 1ull*(s[i]-'a')*f[i]+q;
            if(p==q){
                now = max(i,now);
            }
        }
        for(int i=s.size()-1;i>now;i--){
            cout << s[i];
        }
        cout << s << endl;
    }
    return 0;