TedEd: Can you solve the rogue AI riddle?

A TedEd by Dan Finkel

An AI called NIM has taken over the world's computer system. You, as the only one who can stop it, has found its secret lab. You are in a raft 25 stories above it, and each story has electrified water. You can lower the water level by 1, 3, or 4 at a time and shut off NIM at level 0. However, because NIM knows you are there, it can also lower the water level. You will take turns, and if NIM gets you to 0, then you will be flushed out. How can you make sure that you win? Remember, NIM has calculated every possible outcome and will take full advantage of every mistake you make. To solve this, you can work backwards. If you color code levels by ones that you be on, you can figure it out. To win, you have to be on 1, 3, or 4 when it is your turn. You can also see that 5 and 6 are winning and 7 is not. After that, you could keep going, but a pattern emerges. It repeats green, red, green, green, green, green, red. So, having identified safe levels, you work your way down and save the day.