leetcode题解-138. Copy List with Random Pointer

题意：深拷贝一个链表，链表除了含有next指针外，还包含一个random指针，该指针指向字符串中的某个节点或者为空。

节点定义为：

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

假设原始链表如下，细线表示next指针，粗线表示random指针，没有画出的指针均指向NULL：

该题有一种巧妙地解法，记住即可。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：

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)

/** * 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));    }}