Light Switches in the Cellar — Puzzle for Interview rounds

Question :

In your cellar there are three light switches in the OFF position. Each switch controls one of three light bulbs on floor above.

You may move any of the switches but you may only go upstairs to inspect the bulbs.

When upstairs, you cannot access the switches. What is the minimum number of times you need to go upstairs to determine the switch for each bulb?

Solution :

Turn light switch 1 to the ON position for a few minutes, then turn it off. Then turn on switch 2, and go upstairs. The light bulb that is off and hot is attached to switch 1. The one that is on is attached to switch 2, and the one that is off and cold is attached to switch 3.

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