138. Copy List with Random Pointer

I used hash map to solve the problem with two rounds.
In first round, copy the list without regard with random pointer but make a map between the old node and new node.
In the second round, fix the random pointer by the hash mapping.

1:  /**  
2:   * Definition for singly-linked list with a random pointer.  
3:   * struct RandomListNode {  
4:   *   int label;  
5:   *   RandomListNode *next, *random;  
6:   *   RandomListNode(int x) : label(x), next(NULL), random(NULL) {}  
7:   * };  
8:   */  
9:  class Solution {  
10:  public:  
11:    RandomListNode *copyRandomList(RandomListNode *head) {  
12:      unordered_map<RandomListNode*, RandomListNode*> mp;  
13:      RandomListNode *dummy = new RandomListNode(-1);  
14:      RandomListNode *cur = head, *res = dummy;  
15:      while (cur) {  
16:        RandomListNode *node = new RandomListNode(cur->label);  
17:        mp[cur] = node;  
18:        res->next = node;  
19:        res = res->next;  
20:        cur = cur->next;  
21:      }  
22:      cur = head;  
23:      res = dummy->next;  
24:      while (cur) {  
25:        if (cur->random != NULL) {  
26:          res->random = mp[cur->random];  
27:        }  
28:        res = res->next;  
29:        cur = cur->next;  
30:      }  
31:      res = dummy->next;  
32:      delete dummy;  
33:      return res;  
34:    }  
35:  };