Copy LinkedList with Random Pointer

易错点分析：

1. p->random == NULL的情况；
2. 先统一设好copy_p的random pointer；再用一个循环将original nodes和copied nodes连起来（否则，当random pointer指向已经“恢复”的部分，会发生错误！）切记！

/**
 * 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:
    RandomListNode *copyRandomList(RandomListNode *head) {
        if ( head == NULL ) return NULL;
        RandomListNode* p = head;
        while ( p != NULL ) {
            RandomListNode* copy_p = new RandomListNode(p->label);
            RandomListNode* next = p->next;
            p->next = copy_p;
            copy_p->next = next;
            p = next;
        }
        p = head;
        RandomListNode *copy_p = p->next;
        while ( true ) {
            if ( p->random == NULL ) copy_p->random = NULL;
            else copy_p->random = p->random->next;
            if ( copy_p->next == NULL ) break;
            p = copy_p->next;
            copy_p = p->next;
        }
        p = head;
        copy_p = p->next;
        RandomListNode* copy_head = copy_p;
        while ( true ) {
            if ( copy_p->next == NULL ) {
                p->next = NULL;
                break;
            }
            RandomListNode *next = copy_p->next;
            RandomListNode *copy_next = next->next;
            p->next = next;
            copy_p->next = copy_next;
            p = p->next;
            copy_p = copy_p->next;
        }
        return copy_head;
    }
};