import java.util.*;

class UserMailMerge {
    public static void main(String[] args) {
        List<String> inArr = List.of(
                "user1->[email protected],[email protected]",
                "user2->[email protected],[email protected]",
                "user3->[email protected],[email protected],[email protected]",
                "user4->[email protected],[email protected],[email protected]",
                "user5->[email protected]");
        List<String> outArr = new ArrayList();

        Map<String, Set<String>> userMailsAsIsMap = new LinkedHashMap<>(); // <user, Set<mail>>
        Map<String, String> mailOfUserMap = new LinkedHashMap<>(); // <mail, user>

        for (String rec : inArr){
            String[] userAndMails = rec.split("->");
            String user = userAndMails[0];
            String[] mails = userAndMails[1].split(",");

            userMailsAsIsMap.put(user, Set.of(mails));

            Set<String> previousUsers = new HashSet();
            Set<String> mailsForAddOrChange = new HashSet();
            for(String mail : mails) {
                mailsForAddOrChange.add(mail);
                if(mailOfUserMap.containsKey(mail)){
                    previousUsers.add(mailOfUserMap.get(mail));
                }
            }
            for(String previousUser : previousUsers){
                for(String mailForChange : userMailsAsIsMap.get(previousUser)){
                    mailsForAddOrChange.add(mailForChange);
                }
            }
            if (!previousUsers.isEmpty()){
                user = previousUsers.iterator().next();
            }
            for (String mailForAddOrChange : mailsForAddOrChange){
                mailOfUserMap.put(mailForAddOrChange, user);
            }
        }

        // fill return resulr
        Map<String, Set<String>> userMailMap = new LinkedHashMap<>(); // <user, Set<mail>>
        for(Map.Entry<String, String> mailUserEntry : mailOfUserMap.entrySet()){
            if (userMailMap.get(mailUserEntry.getValue()) == null){
                userMailMap.put(mailUserEntry.getValue(), new LinkedHashSet<>());
            }
            userMailMap.get(mailUserEntry.getValue()).add(mailUserEntry.getKey());
        }
        for(Map.Entry<String, Set<String>> mailUserEntry : userMailMap.entrySet()){
            StringBuilder sb = new StringBuilder(mailUserEntry.getKey());
            sb.append("->");
            sb.append(String.join(",", mailUserEntry.getValue()));
            outArr.add(sb.toString());
        }

        System.out.println("--- inArr: " + inArr);
        inArr.forEach(System.out::println);

        System.out.println("--- outArr: " + outArr);
        outArr.forEach(System.out::println);
    }
}