rt，赛时是 AC 了的，然后被 Hack 掉了，但是是 WA 掉的

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N = 1e5 + 5;
int n, cnt, b;
vector<int> a;
map<string, int> mp;
string k[N];
bool flag;
signed main()
{
    cin >> n;
    for(int i = 1; i <= n; i++)
    {
        string S;
        cin >> S;
        if(S == "start")
        {
            // cout << S << " ";
            if(a.empty())
            {
                cout << "Error\n";
                // continue;
            }
            else if(a.size() == 1)
                cout << k[a[0]] << "\n", b = 1;
            else
            {
                if(flag)
                {
                    // cout << b << "\n";
                    for(int j = 0; j < b; j++)
                    {
                        // vector<int>::iterator iter = find(begin(a), end(a), a[j]);
                        a.push_back(a[0]);
                        a.erase(a.begin());
                    }
                }
                cout << k[a[0]] << " " << k[a[1]] << "\n";
                b = 2;
            }
            flag = true;
        }
        else if(S == "arrive")
        {
            // cout << S << " ";
            cin >> S;
            int s;
            if(!mp[S])
                mp[S] = ++cnt, k[cnt] = S;
            s = mp[S];
            vector<int>::iterator iter = find(a.begin(), a.end(), s);
            if(iter == a.end())
                cout << "OK\n", a.push_back(s);
            else
                cout << "Error\n";
        }
        else
        {
            // cout << S << " ";
            cin >> S;
            int s;
            if(!mp[S])
                mp[S] = ++cnt, k[cnt] = S;
            s = mp[S];
            vector<int>::iterator iter = find(begin(a), end(a), s);
            int id = iter - begin(a);
            if(id == 0 || id == 1 || iter == a.end())
                cout << "Error\n";
            else
                cout << "OK\n", a.erase(iter);
        }
        // for(auto l : a)
        //     cout << k[l] << " ";
        // cout << "\n";
    }
    return 0;
}