unordered_map<Node*,Node*> dict;

    Node* copyRandomList(Node* head) {

        if (!head) return head;

        if (dict.count(head)) return dict[head];

        dict[head]=new Node(head->val, nullptr, nullptr);

        dict[head]->next=copyRandomList(head->next);

        dict[head]->random=copyRandomList(head->random);

        return dict[head];

    }

    Node* copyRandomList(Node* head)

    {

        if (!head) return head;

        unordered_map<Node*,Node*> m;

        Node *p=head;

        while(p)    // make a copy of nodes

        {

            m[p]=new Node(p->val,nullptr,nullptr);

            p=p->next;

        }

        p=head;

        while(p)    // link everyone and fill the random field

        {

            m[p]->next=m[p->next];

            m[p]->random=m[p->random];

            p=p->next;

        }

        return m[head];

    }