Leetcode Copy List with Random Pointer 拷貝鏈表

題目：

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：

1. 遍歷鏈表，拷貝鏈表的每一個節點，並用HashMap記錄原來的節點和拷貝的節點的對應關係。

2. 再次遍歷鏈表，把拷貝的節點連接起來，並加上random節點的關係。

Java代碼實現：

/**
 * 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 head;
            
        HashMap<RandomListNode, RandomListNode> nodes = new HashMap<RandomListNode, RandomListNode>();
        RandomListNode node = head;
        RandomListNode newHead = new RandomListNode(head.label);
        nodes.put(node, newHead);
        node = node.next;
        while(node!=null)
        {
            nodes.put(node, new RandomListNode(node.label));
            node = node.next;
        }
        node = head;
        RandomListNode temp = newHead;
        temp.random = nodes.get(node.random);
        while(node!=null)
        {
            temp.next = nodes.get(node.next);
            node = node.next;
            temp = temp.next;
            if(temp!=null)
                temp.random = nodes.get(node.random);
        }
        return newHead;
    }
}