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.

分析：

这题的关键其实是如何copy random pointer，找到一个random pointer指向的node, time complexity is O(n). 所以总的复杂度在O(n^2).

这里有一个比较巧妙的方法。在每个Node后面添加一个Node，新node的值和前一个Node一样。这样的话random pointer就很容易copy了，

 /**
  * Definition for singly-linked list with a random pointer.
  * class RandomListNode {
  *     int label;
  *     RandomListNode next, random;
  *     RandomListNode(int x) { this.label = x; }
  * };
  */
 public class Solution {
     /**
      * @param head: The head of linked list with a random pointer.
      * @return: A new head of a deep copy of the list.
      */
     public RandomListNode copyRandomList(RandomListNode listHead) {
         if (listHead == null) return listHead;
         RandomListNode current = listHead;
         // add duplicate nodes
         while(current != null) {
             RandomListNode node = new RandomListNode(current.label);
             node.next = current.next;
             current.next = node;
             current = current.next.next;
         }

         // set random pointer
         current = listHead;
         while (current != null) {
             if (current.random != null) {
                 current.next.random = current.random.next;
             }
             current = current.next.next;
         }

         // restore and detach
         RandomListNode newHead = listHead.next;
         RandomListNode current1 = listHead;
         RandomListNode current2 = newHead;
         while (current1 != null) {
             current1.next = current2.next;
             current1 = current1.next;
41             if (current1 != null) {  // very important, cannot remove it.
42                 current2.next = current1.next;
43             }
             current2 = current2.next;
         }
         return newHead;
     }
 }

另一种更好的方法：利用hashmap保存original和copy

 /**
  * Definition for singly-linked list with a random pointer.
  * class RandomListNode {
  *     int label;
  *     RandomListNode next, random;
  *     RandomListNode(int x) { this.label = x; }
  * };
  */
 public class Solution {
     public RandomListNode copyRandomList(RandomListNode head) {
         if (head == null) return null;

         Map<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
         RandomListNode current = head;
         while(current != null) {
             map.put(current, new RandomListNode(current.label));
             current = current.next;
         }

         RandomListNode newHead = map.get(head);

         while(head != null) {
             RandomListNode temp = map.get(head);
             temp.next = map.get(head.next);
             temp.random = map.get(head.random);
             head = head.next;
         }
         return newHead;
     }
 }

转载请注明出处：cnblogs.com/beiyeqingteng/