A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.

Return a deep copy of the list.
1.用map，空间复杂度O(n),时间复杂度O(n)
2.把N’连接到N后面，方便设置random，之后分离N和N’,空间复杂度O(1),时间复杂度O(n)

public RandomListNode copyRandomList(RandomListNode head) {
            if(head == null) return null;
            HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
            RandomListNode dummy = new RandomListNode(0);
            RandomListNode tail = dummy;
            RandomListNode n = head;
            while(n != null){
                tail.next = new RandomListNode(n.label);
                tail = tail.next;
                map.put(n, tail);
                n = n.next;
            }
            tail = dummy.next;
            while(tail != null){
                tail.random = map.get(head.random);
                tail = tail.next;
                head = head.next;
            }
            return dummy.next;
        }



public RandomListNode copyRandomList(RandomListNode head) {
        RandomListNode iter = head, next;

        // First round: make copy of each node,
        // and link them together side-by-side in a single list.
        while (iter != null) {
            next = iter.next;

            RandomListNode copy = new RandomListNode(iter.label);
            iter.next = copy;
            copy.next = next;

            iter = next;
        }

        // Second round: assign random pointers for the copy nodes.
        iter = head;
        while (iter != null) {
            if (iter.random != null) {
            iter.next.random = iter.random.next;
            }
            iter = iter.next.next;
        }

        // Third round: restore the original list, and extract the copy list.
        iter = head;
        RandomListNode pseudoHead = new RandomListNode(0);
        RandomListNode copy, copyIter = pseudoHead;

        while (iter != null) {
            next = iter.next.next;

        // extract the copy
            copy = iter.next;
            copyIter.next = copy;
            copyIter = copy;

        // restore the original list
            iter.next = next;

            iter = next;
        }

        return pseudoHead.next;
        }