两趟循环，第一轮复制节点和next，并建立新旧节点之间的映射。
第二轮复制random

/**
 * Definition for singly-linked list with a random pointer.
 * struct ListNode {
 *     int val;
 *     ListNode *next, *random;
 *     ListNode(int x) : val(x), next(NULL), random(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *copyRandomList(ListNode *head) {
        if(head == NULL)return NULL;
        unordered_map<ListNode*, ListNode*> map;
        ListNode* dummy = new ListNode(0x7f7f7f7f);
        ListNode* cur = dummy;
        ListNode* head_backup = head;
        while(head != NULL){
            ListNode* p = new ListNode(head->val);
            cur->next = p;
            cur = cur->next;
            map[head] = cur;
            head = head->next;
        }
        cur = dummy->next;
        while(head_backup){
            cur->random = map[head_backup->random];
            cur = cur->next;
            head_backup = head_backup->next;
        }

        return dummy->next;
    }
};