Accounts Merge
Using Graphs and DSU

function accountsMerge(accounts) {
  const parent = {};
  const emailToName = {};

  function find(x) {
    if (parent[x] !== x) {
      parent[x] = find(parent[x]);
    }
    return parent[x];
  }

  function union(x, y) {
    parent[find(x)] = find(y);
  }

  for (const account of accounts) {
    const [name, ...emails] = account;
    for (const email of emails) {
      if (!parent[email]) parent[email] = email;
      emailToName[email] = name;
      union(emails[0], email);
    }
  }

  const unions = {};
  for (const email of Object.keys(parent)) {
    const root = find(email);
    if (!unions[root]) unions[root] = [];
    unions[root].push(email);
  }

  const result = [];
  for (const emails of Object.values(unions)) {
    emails.sort();
    result.push([emailToName[emails[0]], ...emails]);
  }

  return result;
}

console.log(accountsMerge([
  ["John", "johnsmith@mail.com", "john00@mail.com"],
  ["John", "johnnybravo@mail.com"],
  ["John", "johnsmith@mail.com", "john_newyork@mail.com"],
  ["Mary", "mary@mail.com"]
]));