Copy List with Random Pointer

跟Clone Graph 的实现细节上有些相似的地方，都要用个map记住已存在结点和新建结点之间的映射。循环两遍，空间O(N).

public class Solution {
    public RandomListNode copyRandomList(RandomListNode head) {
        if(head == null) return null;
        RandomListNode newHead = new RandomListNode(head.label);
        HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
        map.put(head, newHead);
        for(RandomListNode node = head; node.next != null; node = node.next) {
            RandomListNode newNext = new RandomListNode(node.next.label);
            map.get(node).next = newNext;
            map.put(node.next, newNext);
        }
        for(RandomListNode node = head; node != null; node = node.next) {
            map.get(node).random = map.get(node.random);
        }
        return newHead;
    }
}

也可以不用HashMap。

public class Solution {
    public RandomListNode copyRandomList(RandomListNode head) {
        if(head == null) return null;
        for(RandomListNode node = head; node != null; node = node.next.next) {
            RandomListNode newNode = new RandomListNode(node.label);
            newNode.next = node.next;
            node.next = newNode;
        }
        for(RandomListNode node = head; node != null; node = node.next.next) {
            if(node.random != null) {
                node.next.random = node.random.next; //这一步是精华     
	    }
        }
        RandomListNode newHead = head.next;
        for(RandomListNode node = head, newNode = newHead; node != null; node = node.next) {
            node.next = newNode.next;
            if(newNode.next != null) {
                newNode.next = newNode.next.next;
            }
            newNode = newNode.next; //别忘了newNode也要往后走一步！ 
	}
        return newHead;
    }
}