The algorithm is wrong. Three people went out of 30 yuan, spent 25 yuan, and the waiter hid 2 yuan, so each person spent 9 yuan, plus the share of 1 yuan, which happened to be 30 yuan. So this dollar was found.

Summary: This puzzle is mainly because it separates 2 yuan money from 27 yuan money. The original algorithm mistakenly thought that the 2 yuan left by the waiter privately was not included in the 27 yuan, so the wrong result was 1 yuan. In fact, this 27 yuan includes the 2 yuan kept privately, plus the 3 yuan returned, and the result is exactly 30 yuan. There is another algorithm: the 9 yuan money spent by each person already includes the 2 yuan hidden by the waiter (that is, the preferential price of 25 yuan+the waiter's possession of 2 yuan =27 yuan =3*9 yuan).