LeetCode - Copy List with Random Pointer

CopyListwithRandomPointer.java

// use hashmap
public RandomListNode copyRandomList(RandomListNode head) {
        if(head == null) return head;
        HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
        RandomListNode cur = head;
        while(cur != null){
            RandomListNode newNode = new RandomListNode(cur.label);
            map.put(cur, newNode);
            cur = cur.next;
        }
        
        for(Map.Entry<RandomListNode, RandomListNode> entry : map.entrySet()){
            RandomListNode node = entry.getKey();
            RandomListNode copyNode = entry.getValue();
            copyNode.next = map.get(node.next);
            copyNode.random = map.get(node.random);
        }
        return map.get(head);
    }
    
   // use hashmap
   public RandomListNode copyRandomList(RandomListNode head) {
        Map<RandomListNode, RandomListNode> map = new HashMap<>();
        RandomListNode p = head;
        RandomListNode dummy = new RandomListNode(0);
        RandomListNode q = dummy;
        while(p != null){
            q.next = new RandomListNode(p.label);
            map.put(p, q.next);
            p = p.next;
            q = q.next;
        }
        p = head;
        q = dummy;
        while(p != null){
            q.next.random = map.get(p.random);
            p = p.next;
            q = q.next;
        }
        return dummy.next;
    }
// constant space complexity
public RandomListNode copyRandomList(RandomListNode head) {
        if(head == null) return null;
        RandomListNode p = head;
        while(p != null){
            RandomListNode newNode = new RandomListNode(p.label);
            RandomListNode next = p.next;
            p.next = newNode;
            newNode.next = next;
            p = next;
        }
        p = head;
        while(p != null){
            p.next.random = (p.random != null) ? p.random.next : null;
            p = p.next.next;
        }
        p = head;
        RandomListNode dummy = new RandomListNode(0);
        RandomListNode pre = dummy;
        while(p != null){
            RandomListNode next = p.next.next;
            pre.next = p.next;
            pre = pre.next;
            p.next = next;
            p = p.next;
        }
        return dummy.next;
    }
// use hashmap
public RandomListNode copyRandomList(RandomListNode head) {
        if(head==null) return null;
        HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
        RandomListNode dummy = new RandomListNode(0);
        RandomListNode pre = dummy;
        RandomListNode  q = head;
        while(head!=null){
            RandomListNode newNode = new RandomListNode(head.label);
            map.put(head, newNode);
            pre.next = newNode;
            pre = pre.next;
            head = head.next;
        }
        RandomListNode newhead = dummy.next;
        while(q != null){
            newhead.random = map.get(q.random);
            newhead = newhead.next;
            q = q.next;
        }
        return dummy.next;
    }