Accounts Merge

import java.util.*;

class Solution {
    private int[] parent;

    public List<List<String>> accountsMerge(List<List<String>> accounts) {
        Map<String, Integer> id = new HashMap<>();
        Map<String, String> owner = new HashMap<>();
        for (List<String> acc : accounts)
            for (int i = 1; i < acc.size(); i++) {
                id.putIfAbsent(acc.get(i), id.size());
                owner.put(acc.get(i), acc.get(0));
            }
        parent = new int[id.size()];
        for (int i = 0; i < parent.length; i++) parent[i] = i;
        for (List<String> acc : accounts)
            for (int i = 2; i < acc.size(); i++)
                parent[find(id.get(acc.get(i)))] = find(id.get(acc.get(1)));
        Map<Integer, TreeSet<String>> groups = new HashMap<>();
        for (String email : id.keySet())
            groups.computeIfAbsent(find(id.get(email)), k -> new TreeSet<>()).add(email);
        List<List<String>> out = new ArrayList<>();
        for (TreeSet<String> emails : groups.values()) {
            List<String> row = new ArrayList<>();
            row.add(owner.get(emails.first()));
            row.addAll(emails);
            out.add(row);
        }
        return out;
    }

    private int find(int x) {
        while (parent[x] != x) { parent[x] = parent[parent[x]]; x = parent[x]; }
        return x;
    }
}