Hi - I've sourced the code below for automatically converting a lead when criteria are met in process builder. I'd like to modify it to:

1) I'd like it to check if the email exists in a contact and merge the lead into that contact.
2) If a contact doesn't exist, but account does (company name match) merge lead into new contact related to account.
3) If neither, let code run as it normally does, create account and contact.  

Here is the code, any help would be greatly appreciated. 

Public class AutoConvertLeads
{
    @InvocableMethod
    public static void LeadAssign(List<Id> LeadIds)
    {
        LeadStatus CLeadStatus= [SELECT Id, MasterLabel FROM LeadStatus WHERE IsConverted=true Limit 1];
        List<Database.LeadConvert> MassLeadconvert = new List<Database.LeadConvert>();
        for(id currentlead: LeadIds){
                Database.LeadConvert Leadconvert = new Database.LeadConvert();
                Leadconvert.setLeadId(currentlead);                
                Leadconvert.setConvertedStatus(CLeadStatus.MasterLabel);
                Leadconvert.setDoNotCreateOpportunity(TRUE); 
                MassLeadconvert.add(Leadconvert);
        }
        
        if (!MassLeadconvert.isEmpty()) {
            List<Database.LeadConvertResult> lcr = Database.convertLead(MassLeadconvert);
        }
    }
}