What could be a system which knows not the meaning of defeat?
If you could take your time at the roulette table,while having some drinks and chatting with friends (or not), if you can afford to invest 1000 (very safe) to 1500 units (invisible),take some notes,then you may have the Holly Grail.
Wait for ALL numbers to come except one,bet the last remaining number till it comes. (maximum 200 betting spins)
It's not like betting from the beginning,usually groups of hot or cold numbers come in clusters,therefore if the number before the last delays,it'd cost only in time and when it comes,the last will be nearby.
From my personal experience,after 3 cycles (111 spins) there are 2 up to 4 numbers which have not appeared already,these remaining could come within 3 additional cycles (222 spins) but it doesn't really matter when you are not betting.
The remaining 2 up to 4 numbers could come up to 150th or 180th spin,but no matter how long would take,the before the last number never is very far from the last.
It's a winning strategy if you have the necessary requirements,you may call it the "Last Number".
But if you lack of patience,discipline,funds you should rather not play it,because mistakes during the game are costing money,remember this!