Another problem of interest in percolation theory is to determine the threshold probability where a liquid will permeate a porous material. One way to model this is to consider the barriers between pairs of adjacent cells in some random order and remove them in this order. At the point when the top and bottom are connected, we would then take the ratio of r/s as an approximation to this threshold probability, where r is the number of barriers considered up to this point and s is the total number of barriers. Describe an algorithm for efficiently computing this threshold value right at the moment it occurs, given a random listing of the pairwise barriers in the porous material as input.