67-set-parent-case

public void linkParent(Case c1, Case c2) {
    //code here
    if(c1.ContactId == c2.ContactId && c1.ContactId != null && c2.ContactId != null){
        if(c1.CreatedDate < c2.CreatedDate){
            c2.ParentId = c1.Id;    
        }
        else if (c2.CreatedDate < c1.CreatedDate){
            c1.ParentId = c2.Id;
        }
        
    }
}