leetcode題解-138. Copy List with Random Pointer

題意：深拷貝一個鏈表，鏈表除了含有next指針外，還包含一個random指針，該指針指向字符串中的某個節點或者爲空。

節點定義爲：

// Definition for singly-linked list.
 public class RandomListNode {
     int label;
     RandomListNode next, random;
     RandomListNode(int x) { this.label = x; }
 }

假設原始鏈表如下，細線表示next指針，粗線表示random指針，沒有畫出的指針均指向NULL：

該題有一種巧妙地解法，記住即可。構建新節點時，指針做如下變化，即把新節點插入到相應的舊節點後面：

分兩步

1、構建新節點random指針：new1->random = old1->random->next, new2-random = NULL, new3-random = NULL, new4->random = old4->random->next

2、恢復原始鏈表以及構建新鏈表：例如old1->next = old1->next->next, new1->next = new1->next->next

該算法時間複雜度O(N)，空間複雜度O(1)

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

    public static RandomListNode copyRandomList(RandomListNode head) {
         if(head == null) return null;
         RandomListNode fastNode = head;
         while(fastNode != null){
             RandomListNode insertNode = new RandomListNode(fastNode.label);
             insertNode.next = fastNode.next;
             fastNode.next = insertNode;
             fastNode = fastNode.next.next;
         }
         fastNode = head;
         while(fastNode != null){
             if(fastNode.random != null){
                 fastNode.next.random = fastNode.random.next;
             }
             fastNode = fastNode.next.next;
         }
         fastNode = head;
         RandomListNode newHead = head.next;
         while(fastNode != null){
             RandomListNode newNode = fastNode.next;
             fastNode.next = newNode.next;
             if(newNode.next != null){
                 newNode.next = newNode.next.next;
             }
             fastNode = fastNode.next;
         }

         return newHead;
    }
    public static void print(RandomListNode head){
        while(head != null){
            System.out.println(head.label);
            head = head.next;
        }

    }
    public static void main(String[] args) {
        RandomListNode l1 = new RandomListNode(1);
        RandomListNode l2 = new RandomListNode(2);
        RandomListNode l3 = new RandomListNode(3);
        RandomListNode l4 = new RandomListNode(4);
        l1.next = l2;
        l2.next = l3;
        l3.next = l4;
        l1.random = l3;
        l4.random = l2;
        print(copyRandomList(l1));
    }
}