copy list with random pointer

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.

解法：

算法很巧妙，不用保存原始链表的映射关系，构建新节点时，指针做如下变化，即把新节点插入到相应的旧节点后面：

同理分两步

 

1、构建新节点random指针：new1->random = old1->random->next, new2-random = NULL, new3-random = NULL, new4->random = old4->random->next

 

2、恢复原始链表以及构建新链表：例如old1->next = old1->next->next,  new1->next = new1->next->next

 

该算法时间复杂度O(N)，空间复杂度O(1)