LeetCode OJ - Copy List with Random Pointer

题目：

　　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.

解题思路：

　　采用递归求解，并利用unordered_map来记录新生成的节点。

代码：

/**
 * Definition for singly-linked list with a random pointer.
 * struct RandomListNode {
 *     int label;
 *     RandomListNode *next, *random;
 *     RandomListNode(int x) : label(x), next(NULL), random(NULL) {}
 * };
 */
class Solution {
public:
    unordered_map<int, RandomListNode*> dict;
    RandomListNode *copyRandomList(RandomListNode *head) {
        if (head == NULL || dict.count(head->label)) return NULL;

        RandomListNode *new_head = new RandomListNode(head->label);
        dict[head->label] = new_head;

        if (head->next != NULL) {
            copyRandomList(head->next);
            new_head->next = dict[head->next->label];
        }
        if (head->random != NULL) {
            copyRandomList(head->random);
            new_head->random = dict[head->random->label];
        }
        return new_head;
    }
};