**Leetcode_copy-list-with-random-pointer (c++ and python version)

地址：http://oj.leetcode.com/problems/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.

这是一道好题目，也是我被面过的一道题。

四个月前程老板用这道题面我的时候，我刚开始还是云里雾里，后来跟程老板交流了一下，磕磕碰碰用了三四个map搞出来，让自己多说两遍自己都会被自己绕晕。程老板说他用这道题面过很多人，其中不乏牛逼的人。

总之祝程老板创业成功。

这道题如果可以画一张图就能一目了然了。

在遍历原始链表的时候，创建一个新的链表，建立原始链表每一个结点与新建结点的关系（这里用了一个map）。这一次遍历解决了next指针。

接下来第二次遍历，某一个节点的random指针，比如p1的random指向px，px与copy链表里新创建的节点通过map找到的是px^, p1与copy里新创节点通过map找到的是p1^，那么p1^的random指向的就是px^。

第17行map插入的时候开始写的是dm.insert(make_pair<p, pnew>). 给报了两次compile error

后来改成了强制转化的形式，但是忘记了加pair。

这样三次compile error了后一次AC了，汗。这样的题应该多做，打上两颗*。

参考代码：

/** * 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:    RandomListNode *copyRandomList(RandomListNode *head) {        map<RandomListNode*, RandomListNode*> dm;        RandomListNode* p = head, *lastp = NULL, *newhead = NULL;        while(p)        {            RandomListNode* pnew = new RandomListNode(p->label);            dm.insert(pair<RandomListNode*, RandomListNode*>(p, pnew));            if(p!=head)            {                lastp->next=pnew;                lastp = lastp->next;            }            else            {                newhead = lastp = pnew;            }            p = p->next;        }        p = head;        while(p)        {            dm[p]->random = dm[p->random];            p = p->next;        }        return newhead;    }};

class Solution {
public:
    RandomListNode *copyRandomList(RandomListNode *head) {
        if(!head)
            return head;
        unordered_map<RandomListNode*, RandomListNode*>mp;
        RandomListNode *p = head, *q = NULL, *pre = NULL;
        while(p)
        {
            q = new RandomListNode(p->label);
            mp[p] = q;
            p = p->next;
            if(pre)
                pre->next = q;
            pre = q;
        }
        p = head;
        while(p)
        {
            mp[p]->random = mp[p->random];
            p = p->next;
        }
        return mp[head];
    }
};

python:

# Definition for singly-linked list with a random pointer.
# class RandomListNode:
#     def __init__(self, x):
#         self.label = x
#         self.next = None
#         self.random = None

class Solution:
    # @param head, a RandomListNode
    # @return a RandomListNode
    def copyRandomList(self, head):
        if not head:
            return head;
        mp = {}
        p = head
        pre = None
        while p:
            q = RandomListNode(p.label)
            mp[p] = q
            if pre:
                pre.next = q
            pre = q
            p = p.next
        p = head
        while p:
            if p.random:
                mp[p].random = mp[p.random]
            else:
                mp[p].random = None
            p = p.next
        return mp[head]