1. 题目

在这里插入图片描述

2. 思路

(1) 递归+HashMap

利用HashMap存储已创建的结点与原结点的映射关系。
递归得到每个结点的next和random，若HashMap中包含该结点对应的新结点，则直接指向新结点即可，否则创建新结点并存入HashMap中。

(2) 迭代

首先遍历链表，创建的新结点插入原结点的后面，然后再遍历链表，根据原结点的random修改新结点的random，最后拆分两个链表，返回新链表即可。

3. 代码

import java.util.HashMap;
import java.util.Map;

public class Test {
    public static void main(String[] args) {
    }
}

class Node {
    int val;
    Node next;
    Node random;

    public Node(int val) {
        this.val = val;
        this.next = null;
        this.random = null;
    }
}

class Solution {
    private Map<Node, Node> map = new HashMap<>();

    public Node copyRandomList(Node head) {
        if (head == null) {
            return null;
        }
        if (map.containsKey(head)) {
            return map.get(head);
        }
        Node newHead = new Node(head.val);
        map.put(head, newHead);
        newHead.next = copyRandomList(head.next);
        newHead.random = copyRandomList(head.random);
        return newHead;
    }
}

class Solution1 {
    public Node copyRandomList(Node head) {
        if (head == null) {
            return null;
        }
        Node pre = head;
        Node cur;
        while (pre != null) {
            cur = new Node(pre.val);
            cur.next = pre.next;
            pre.next = cur;
            pre = cur.next;
        }
        pre = head;
        while (pre != null) {
            cur = pre.next;
            if (pre.random != null) {
                cur.random = pre.random.next;
            }
            pre = cur.next;
        }
        Node newHead = head.next;
        pre = head;
        while (pre != null) {
            cur = pre.next;
            pre.next = cur.next;
            if (pre.next != null) {
                cur.next = pre.next.next;
            } else {
                cur.next = null;
            }
            pre = pre.next;
        }
        return newHead;
    }
}