350. Among 18 coins, one is counterfeit. Genuine coins weigh the same, while the counterfeit coin differs in weight from the genuine ones. What is the minimum number of weighings on correct balance scales without weights needed to determine whether the counterfeit coin is lighter or heavier than the genuine ones? (There is no need to find the counterfeit coin.)
$\triangle$ Let's number the coins. Divide the set of coins into three piles, with 6 coins in each.

For the first weighing, place all the coins from the first pile on one scale pan, and all the coins from the second pile on the other. There are two possible cases.

1) Suppose the scales balance during this weighing. Then the counterfeit coin is in the third pile.

Now place the first pile of coins on one scale pan, and the third pile on the other. If, for example, the third pile tips the scales, then the counterfeit coin is heavier than the genuine ones.

2) Suppose the scales were unbalanced during the first weighing. Then the counterfeit coin is either in the first or the second pile. Therefore, all the coins in the third pile are genuine.

Place the first pile of coins on one scale pan, and the third pile on the other. If the scales are unbalanced, then the counterfeit coin is in the first pile, and the last weighing
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will show whether it is lighter or heavier than the genuine ones. If the scales balance, then the counterfeit coin is in the second pile, and the first weighing will also determine whether it is lighter or heavier than the genuine ones.

Answer: in two.