# Problem 138: Copy List with Random Pointer

> [https://leetcode.com/problems/copy-list-with-random-pointer/](https://leetcode.com/problems/copy-list-with-random-pointer/)

## 思路

* 这道题的难点就在于，不知道某个点之前存了没有。所以我们可以用HashMap来存储这样的映射关系
* 用newNode来记录每一个新存入的点。有就直接get，没有就put到map里面。

```java
/**
 * 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;
        }

        RandomListNode dummy = new RandomListNode(0);
        RandomListNode pre = dummy;
        RandomListNode newNode;
        HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
        while (head != null) {
            if (map.containsKey(head)) {
                newNode = map.get(head);
            } else {
                newNode = new RandomListNode(head.label);
                map.put(head, newNode);
            }
            pre.next = newNode;

            if (head.random != null) {
                if (map.containsKey(head.random)) {
                    newNode.random = map.get(head.random);
                } else {
                    newNode.random = new RandomListNode(head.random.label);
                    map.put(head.random, newNode.random);
                }
            }

            pre = newNode;
            head = head.next;
        }

        return dummy.next;
    }
}
```

## 易错点

1.  corner cases的情况

    ```java
    if (head == null) {
        return null;
    }
    ```

    之前多此一举地考虑了`head.next == null`的情况，返回`head`出错。
2.  node 和 random 关系分别处理

    ```java
    if (map.containsKey(head)) {
        newNode = map.get(head);
    } else {
        newNode = new RandomListNode(head.label);
        map.put(head, newNode);
    }
    pre.next = newNode;
    ```

    ```java
    if (head.random != null) {
         if (map.containsKey(head.random)) {
              newNode.random = map.get(head.random);
         } else {
              newNode.random = new RandomListNode(head.random.label);
              map.put(head.random, newNode.random);
         }
    }
    ```
