Merges the sets containing `first_elem` and `sec_elem` using union by size. The smaller set is always attached to the root of the larger set to ensure balanced trees. # Parameters - `first_elem`: The first element whose set is to be merged. - `sec_elem`: The second element whose set is to be merged. # Returns The root of the merged set, or `usize::MAX` if both elements are already in the same
(&mut self, first_elem: usize, sec_elem: usize)
source not stored for this graph (policy: none)