3种版本的答案,第一种使用virtual top and bottom site, 但有backwash的问题,解决这个问题有两种方法: 1. 使用2个WQUUF, 但会增加memory. One for checking if the system percolates(include virtual top and bottom), and the other to check if a given cell is full(only include virtual top). 而且要注意,…