checkIn(id, stationName, t) {
    this.stationSystem[id] = [stationName, '', t, ''];
  }
Time O(1) | Space O(1)
Records the check-in time and station for a user.
@param {number} id - User ID
@param {string} stationName - Check-in station name
@param {number} t - Check-in time
@return {void}