Leetcode 138 Copy List with Random Pointer

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.

复制一个带有随机后续节点的链表，

因为随机后续节点可能在复制到当前结点时还没有被创建出来，所以需要保存新节点和老节点之间的对应关系。

当然也有人通过在老节点后插入新节点的方法完成，这样的好处是空间复杂度降低了，但坏处是实现起来麻烦一些，而且如果链表中有环，这样的方法将失效。

/**
 * 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) {
        unordered_map <RandomListNode*, RandomListNode*> mp;
        if(!head) return NULL;
        RandomListNode* p = head;
        RandomListNode* res = new RandomListNode(head->label);
        RandomListNode* q = res;
        mp[p] = q;
        while(p->next)
        {
            p = p->next;
            q->next = new RandomListNode(p->label);
            q = q->next;
            mp[p] = q;
        }
        p = head;
        q = res;
        while(p)
        {
            q->random = mp[p->random];
            p = p->next;
            q = q->next;
        }
        return res;
    }
};