There are n stacks of n identical-looking coins. All of the coins in some of these stacks are genuine except for one which is counterfeit, while all the coins in the other stacks are genuine. Every genuine coin weighs 10 grams; every fake weighs 11 grams. You have an analytical scale that can determine the exact weight of any number of coins.

1. Devise a brute-force algorithm to identify the stacks with the fake coins and determine its worst-case efficiency class.

2. What is the minimum number of weighings needed to identify the stack with the fake coins?