67-set-parent-case.apex

public void linkParent(Case c1, Case c2) {

    if((c1 <> null && c2 <> null) && (c1.ContactId <> null && c2.ContactId <> null) &&
        c1.ContactId == c2.ContactId){
            if(c1.CreatedDate > c2.CreatedDate){
                c1.ParentId = c2.Id;
            } else if(c1.CreatedDate < c2.CreatedDate){
                c2.ParentId = c1.Id;
            }
        }

}