pub fn trees(&mut self) -> Vec<Vec<usize>> {
        use std::collections::HashMap;

        // Maps a tree root to the index of the set
        // containing its children
        let mut root_sets: HashMap<usize, usize> = HashMap::new();

        let mut sets: Vec<Vec<usize>> = vec![];
        for i in 0..self.count {
            let root = self.root(i);
            match root_sets.get(&root).cloned() {
                Some(set_idx) => {
                    sets[set_idx].push(i);
                }
                None => {
                    let idx = sets.len();
                    let set = vec![i];
                    sets.push(set);
                    root_sets.insert(root, idx);
                }
            }
        }
        sets
    }
Returns the elements of each tree.
src/union_find.rs