Trigger roundrobintrigger on Lead (Before Insert){
    //I have Created number field storing round roubin number based on xyz field
    // We can use here either custom metadata or custom setting for declaring pre values of persons.
    //like we have 20 people in SDRs pre values are persion1=>1,Person2=>2,Person3=>3....etc
    //using this values we allocating users and WHY Custom metadata or custom settings is users may come or leaving any time.,
    //so if you do not want any on queue you can remove from either custom metadata or cutom settings.
    // Now I'm using Custom Settings (Type = List) and I created one custom field called round roin id(pre value) //and while creating records here insted of using user name use userid.
    //Now Querying customsettings records and puuting in map based rounrobin id
    Map<Integer,String>SDRsusersMap =New Map<Integer,String>();
    For(SDRsusersList__c sdr:[Select id,Name,Round_roubin_Id__c from SDRsusersList__c]){
        SDRsusersMap.put(sdr.Round_roubin_Id__c,sdr.name);//Name is nothing but a user id.
    }
    // similarly for AccountExecs group
    Map<Integer,String>AccountExecsusersMap =New Map<Integer,String>();
    For(AccountExecsusersList__c acc:[Select id,Name,Round_roubin_Id__c from AccountExecsusersList__c]){
        AccountExecsusersMap.put(acc.Round_roubin_Id__c,acc.name);//Name is nothing but a user id.
    }
    Boolean isxyzexist=false;
    Boolean notisxyzexist=false;
    for(Lead l:Trigger.New){
        if(l.isxyzexist__c!=null){
            isxyzexist=true;
        }
        else {
            notisxyzexist=true;
        }
    }
    
    List<Lead> SDRsList=new List<Lead>();
    List<Lead> AccountExecsList=new List<Lead>();
    Integer SDRsnextNumber=0;
    Integer AccountExecsnextNumber=0;
    if(isxyzexist){
        SDRsList = [Select id,Round_roubin_Id__c from Lead where 
                    Round_roubin_Id__c != null and isxyzexist__c!=null order by Round_roubin_Id__c desc limit 1];
        if(SDRsList!=null && !SDRsList.isEmpty()){
            if(SDRsList[0].Round_roubin_Id__c !=null)
                SDRsnextNumber=Integer.valueOf(SDRsList[0].Round_roubin_Id__c );
        }
    }
    if(notisxyzexist){
        AccountExecsList = [Select id,Round_roubin_Id__c from Case where 
                            Round_roubin_Id__c != null and isxyzexist__c=null order by Round_roubin_Id__c desc limit 1];
        
        if(AccountExecsList!=null && !AccountExecsList.isEmpty()){
            if(AccountExecsList[0].Round_roubin_Id__c !=null)
                AccountExecsnextNumber=Integer.valueOf(AccountExecsList[0].Round_roubin_Id__c );
        }
    }
    for(Lead it : Trigger.New){
        if(it.isxyzexist__c !=Null){
            if(it.Round_roubin_Id__c == null){
                SDRsnextNumber=SDRsnextNumber+1;
                it.Round_roubin_Id__c = SDRsnextNumber;
                it.OwnerId=SDRsusersMap.get(SDRsnextNumber);
                if(SDRsnextNumber==20)//why Im assuming for 20 members so if reached again restarting..you can put //what number u want.
                    SDRsnextNumber=0;
            }  
        } 
        else if(it.isxyzexist__c ==Null){
            if(it.Round_roubin_Id__c == null){
                AccountExecsnextNumber = AccountExecsnextNumber+1; 
                it.Round_roubin_Id__c = AccountExecsnextNumber;
                it.OwnerId=AccountExecsusersMap.get(AccountExecsnextNumber);
                if(AccountExecsnextNumber==20)//This users count also no need to give hard code you can declare //size of that map like AccountExecsnextNumber==AccountExecsusersMap.size() this will give count of users in that //group
                    AccountExecsnextNumber=0;
            } 
        } 
    } 
}